I'm guessing this is in response to the basement square footage question?
You determine a basement square footage estimate only after doing it the other way and after awhile just converting the bottom line numbers to a per square foot number that works out pretty close to the original way which is adding up all the expenses.
Totally simplified for example only:
1000 sq foot basement
drywall - 30 sheets @ 6 each = $180
nails for drywall = $21
tape = $13
mud = $34
labor = $400
------------------
Total $648
Add all the other stuff - demo, trash removal, elec, plumbing, framing.....
say you get a total of: 1000 sq foot basement = $12,000 to do the job and make the profit you need to run your businees.
$12,000 divided by 1000 square foot = $120 a foot.
Next basement is 2500 square feet so 2500 sq ft x $120 = $30,000
Will this work every time? Hell no. Somebody throws in they want a bar built or wants a wall guilded in gold leaf you're gonna have to make an adjustment, but that is what allowances are for, right?