You would let the computer calculate two different tents. The first, the outer tent is bigger, the other tent would simply be two inches smaller.
Before this spreadsheet I drew a very accurate picture of the tent and took my measurements from there. The problem was that it would take forever and if you dicided half way that you wanted your tent two inches higher, you would have to start allover again. Now every dome tent you could imagine is just a couple of keypresses away.

One of my earlier tents (done by "hand")could be viewed here:

tent
The method was basically the same, just not computerized.