Thanks again for sharing this program with us! I finally took the time to play with it, and it seems pretty easy and straightforward.

One thing I was thinking about is how to use this with a trapezoidal instead of rectangular floor plan. Quickly trying to wrap my head around it, my thought was:

For X1/Y1 measurements (end panels of tent)
Head End (Wider): enter all dimensions and desired width of head end- use X/Y coordinates provided

Foot end (narrower): change width to match foot end width, all other specs stay the same- use new set of X/Y coordinates provided

For X2/Y2 measurements (long side panels of tent)
Length: Since you no longer have a rectangle, using the current input specs wouldn't work (the actual length of the tent side is longer). But all you need to do is determine the current length of the side (simple pythagorean theorum stuff):

(length of tent from step one)2 + (1/2*(head width-foot width))2 = (current length of side)2

(sorry, can't get it to superscript, so the "2" in each part is "squared")

A little calculator work and you are done. Does this sound about right to you?

I think you might get by with your method. There is definitely a difference in length between the broad and the thinner side at the "corners", so the "big" panel sits a bit askiew in-between.
A few irregularities here and there are usually no problem and are compensated by fabric stretch.
But please keep in mind that this is definitely just an aproximation. While the spreadsheet- used for the intended purpose, a square tent- is accurate to the letter, using it for a tapered tent isn't. Please use a paper model and even a "cheap fabric prototype" before you sacrifice your expensive fabric.

Not only square tents could be calculated with the spreadsheet, also polygonal dome tents (regular polygon)are possible.
The tent width is the length of one polygon segment, the tent lenght is two times the apothem of the polygon.
Then use the values of X1 and Y1 to cut out your fabric.