Hi! I just tried your formula, and the results are very close to the calculator indeed! The correct formula is however a lot simpler All formulas are pulled from a database and calculated serverside as you figured out.
Unreal 3 games are pretty much all the same, but some are not affected by FOV, and some, like BioShock Infinite, are completely different.
I will make an explanation of some common formulas (like the well known Source engine one and Unreal 3) in an article later, but as an answer to you, the Unreal 3 forumla is simply:
Distance (inches) = ((65536/SENS)*(DFOV/FOV))/DPI
DFOV = Default FOV (usually 90).
Hope this helps