Autopopulate EntityReference field using rules? I'd like to have an EntityReference field be autopopulated by the node that was just navigated from.

So the workflow would be:

User views page for Business A -> User clicks review link -> User is presented with review form, &amp; EntityReference field is automatically populated with the name/id of Business A.

What I don't know is which action to designate to tell Rules to do this. I have Event set to "before saving content," and Elements set to
"Entity has field, Parameter: Entity: [node], Field: field_venue_reference." These seem like the correct settings for these portions from what I have read, but since I'm new to Rules, I'm not completely sure of that either. Any suggestions?
You want the Entity Reference Prepopulate module.  It's very similar to the Node Reference URL module but the maintainer of that module is stubborn and sticking to only referencing Nodes and not Entities.

Rules has always had a hard time with prepopulating reference fields so please don't go that route.  You can combine Rules and ERP by using Rules to populate other fields and ERP to handle the referencing.

Everything is URL based so you must setup your links appropriately.  The structure is node/add/article?[field_ref]=[id]
So you could have
node/add/review?field_business_ref=457

your link to the form could look something like l('add review', 'node/add/review?field_business_ref='. $node->id)