I need some help with the "custom geometry calculator"

I have been trying to determine the stack and reach for my 2006 Litespeed Tachyon 700c tri-bike. I have read the many articles relating to this and have mustered up the courage to attempt the usage of the “custom geometry calculator”. I have a handle on all the necessary data with the exception of the last value termed “rear wheel”. How do I determine this? Is this something standard or something measured or both…Help!
Thanks,
Coop

I think I figured it out: it is the wheelbase minus the front/center. That seems to make sense but is it correct?
Thanks again.
Coop