How to calculate rigid fork replacement for suspension one?
If you want a rigid fork for a 29er with shocks just buy a fork for a suspension corrected frame.  If the fork is listed for a mountain bike then it is (probably) suspension corrected.  I suspect that about any rigid fork that will take a 2" tire will be suspension corrected.   

Measuring is not going to do you much good as you can't buy custom lengths (that I am aware of).

Below is a suspension corrected rigid - see the space at the top:
 

For a workout bike I don't get why you need 2" tires?  You are going to have trouble finding road 2" tires.  There are some rigid mountain bikes (Niner Ross 9 Plus) and also the Salsa Fargo.  Another option would be to buy a used mountain bike priced right because of a worn out shock.