Using a Variable in a Rules Calculation
I ended up using a slightly different approach. Just in case somebody else is interested in this I'm posting my final solution here.

PROBLEM  

"Estimate" a ship date based on the turnaround times of all the products in cart whenever a product is added or removed from the cart and at checkout (just in case the customer decides to check out days after adding the products).

Additional modules and fields:


Using the interval field module I added a "Turnaround Time" field to the product variation type.
I also added a date field to the order entity - "field-due-date".


RULES

I created 3 items under rules.

Reaction Rule: Set the estimated shipping date

+ Events
    + After adding a product to the cart
    + After removing a product from the cart
    + Completing the checkout process
+ Actions
    + rule set: Set the estimated shipping date
        // these settings ensure that you run through each and every line item, instead of just the current item; spend a long time doing that - over and over. :(
        + Line Item
          Data Selector: commerce-order:commerce-line-items:0
        + Order
          Data Selector: commerce-order
        + Commerce Product
          Data Selector: commerce-product


Ruleset: Get Line Item Turnaround Value
rules_get_line_item_turnaround_value
This rule gets the integer value of each line-item's turnaround time (in days in my case) and writes it to "line-item-turnaround-time" variable which is passed along to the next ruleset. I could probably have used a regular Rule instead of a Rule Set here.

Date Type
    Commerce Line Item, Line Item, line_item, Parameter
    Commerce Order, Order, order, Parameter
    Commerce Product, Product, product, Parameter
    Integer, Line Item Turnaround Time, line_item_turnaround_time, provided

+ Rule: Get the Line Item Turnaround Time as a Number
  Provides variables: Line Item Turnaround (line_item_turnaround)
    + Conditions
        + Entity has field
          Parameter: Entity: [line-item], Field: commerce_product
          //exposes the line item to the rule
        + Entity has field
          Parameter: Entity: [line-item:commerce-product], Field: field_turnaround_time
          //exposes the line item's product variation to the rule
    + Elements
        + Add a variable
          Parameter: Type: Integer, Value: [line-item:commerce-product:field-turnaround-time:interval]
          Provides variables: Line Item Turnaround (line_item_turnaround)
          //pulls the turnaround time as an integer
        + Set a data value
          Parameter: Data: [line_item_turnaround_time], Value: [line-item-turnaround]
          //write's the turnaround time to the "line_item_turnaround_time" variable


Ruleset: Set the estimated shipping date
rules_set_the_estimated_shipping_date
This rule pulls the turnaround time from a list of the line items in the current order, compares them and uses the largest value to adjust the estimated delivery date. Again, I could probably have used a regular Rule here instead of a Rule Set.

Date Type
    Commerce Line Item, Line Item, line_item, Parameter
    Commerce Order, Order, order, Parameter
    Commerce Product, Product, product, Parameter

+ Rule: Calculate the estimated shipping date
  Provides variables: Order Turnaround Time (order_turnaround_time), Conversion result (conversion_result), New Estimated Shipping Date (new_estimated_shipping_date)
    + Conditions
        + Entity has field
          Parameter: Entity: [line-item], Field: commerce_product
          //exposes the line item to the rule
        + Entity has field
          Parameter: Entity: [line-item:commerce-product], Field: field_turnaround_time
          //exposes the line item's product variation to the rule
        + Entity has field
          Parameter: Entity: [order], Field: field_due_date
          //exposes the current order to the rule
    + Elements
        + Set a data value
          Parameter: Data: [order:field-due-date], Value: [site:current-date]
          //set the order's due date to the site's current date. This does two things: 1. ensures that the shipping date is based on the exact time the order is completed, as opposed to a previously calculated value, or the order creation date. 2. rules fail if no value is set
        + Add a variable
          Parameter: Type: Decimal number, Value: 0
          Provides variables: Order Turnaround Time (order_turnaround_time)
          //set the base turnaround time to 0 so we have something to compare our line item turnaround times to
        + Loop
          Parameter: List: [order:commerce-line-items]
          List item: List of Line Items (list_of_line_items)
          //loop through a list of the current order's line items
            + rule set: Get Line Item Turnaround Value
              Parameter: Line Item: [list-of-line-items], Order: [list-of-line-items:order], Product Display: [line-item:commerce-product]
              Provides variables: Line Item Turnaround Time (line_item_turnaround_time)
              //runs the "Get Line Item Turnaround Value" and pulls the "line_item_turnaround_time" for the current loop line item
            + Calculate a value
              Parameter: Input value 1: [line-item-turnaround-time], Operator: max, Input value 2: [order-turnaround-time]
              Provides variables: Calculation result (result)
              //check to see if the current loop line item's turnaround time is larger than the value of the order-turnaround-time
            + Set a data value
              Parameter: Data: [order-turnaround-time], Value: [result]
              //if the current loop line item's turnaround time is larger than the value of the order-turnaround-time write it as the new "order-turnaround-time" otherwise keep the old value
        + Convert data type
          Parameter: Target type: Integer, Value to convert: [order-turnaround-time], Rounding behavior: Always up (9.5 -&gt; 10)
          Provides variables: Conversion result (conversion_result)
          //converts the order-turnaround-time to an integer, just in case it's not, so we can apply it as an interval
        + Apply an interval to a date
          Parameter: Interval number: [conversion-result], Interval period: Days, Date: now
          Provides variables: New Estimated Shipping Date (new_estimated_shipping_date)
          //add the new "order-turnaround-time" to the "site:current-date[field-due-date]" to calculate the estimated shipping date
        + Set a data value
          Parameter: Data: [order:field-due-date], Value: [new-estimated-shipping-date]
          //write the new estimated shipping date to the field-due-date field on the order


Hope that saves someone a couple of hours.