a nice link for this
http://www.nwmarinedesign.com/sadscrip.htm
Probably too late but make sure you are using SA as measured with a 100% foretriangle - that is the standard and the only way you can compare to other boats. Also use your real displacement for real numbers, not the factory delivered weight (unless you are just comparing fantasy numbers).
Using the actual SA - say with a 150% genoa might give you and warm and smug feeling, but then you can't compare with other boats.
Note some manufactures/sales people will cheat the numbers to make their boats look better.
As an example my boat (Gemini 105mc)
with 100% foretriangle only has 590 sq ft- 20.5 sa/d
with 150% (670 sq ft) - 23.3 sa/d
with screacher (780 sq ft) - 27.1 sa/d
I'm pretty sure all the sq ft measurements on mine include the roach on the main, so as you can see, it's apples and crab apples.