Could you drop me a note when you have a working program or sheet? I would like to try to play with it abit if you don't mind. But I generally like what I see.
Heck, I'll post it to COTI, though it's not pretty and it is very very loose. You'll need to install Perl on your computer too. At least until I port it to the 'web.
It does have a useful estimation routine that figures out the percentage of volume dedicated to the variable elements of the ship, and back-figure volume therefore based on the payload (weapons and defenses and their support).
So I select performance and TL, then select weapons and defenses, type "estimate", and it estimates the tonnage and cost. Very handy in a rule-of-thumb way.
A spreadsheet would have indeed been a better choice. But it's not as fun to write.