How to calculate rigid fork replacement for suspension one? Let's say I am reading spec for 29er with suspension fork and I would like to calculate replacement parameters for rigid one. Calculation is needed because I don't want to change geometry after replacement -- so the rigid fork should feel (in sense of geometry) as locked in neutral position suspension fork.

How to do the calculation, what parameters are important? Example:


  "Custom SR Suntour XCT 29, w/ custom Multi-Circuit Damping, alloy
  lowers, coil/MCU spring, Hi-Ten 28mm stanchions, 1-1/8" steerer,
  hydraulic lockout w/ preload adjust, disc mount, 100mm travel"


(text from Specialized Rockhopper 29).

Background 1: I want to do calculations before I buy the bike to avoid situation, that I am buying popular bike but there is no rigid fork replacement (or it is very hard to get).

Background 2: In reality I wanted to buy fitness bike (100% rigid, disc brakes, 700C) and mount wider tires (2"). After some query it appears that all fitness bikes (like Trek 7.2 FX, Cube Hyde, etc) have so little space that you can fit 1.75" tire max (Kross Seto). I found only single bike so far with frame for wider tires (Sulry Ogre), but it is out of reach for me (Poland). So I see no other option as do more expensive replacement -- instead of buying fixed fork bike and replacing tires, buying 29er and replacing fork.
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.