I finished my calculator, using that site's equations. It offers two modes, simple and advanced.
In simple, you enter your guitar's scale length and desired tension (I think most people use around 15 or 16 pounds), how much you want to downtune(or not), and whether you want dropped tuning (6th string a step lower), then click solve and it'll show you the gauges you would need (approximately, since the calculator can't be exact due to different brands using different density strings and such). It works for 7 strings, too (though the drop tuning option only effects the 6th string).
In advanced, you can enter the note frequency (or select it from the list, reccommended) and get just a single string. This is good if you want to use some wacko tuning. To select a frequency from the list, click on the desired string (B,E,A,D,G,B, or E) and the list will refresh and show the steps down and frequencies and such. If you click the note on the list, it'll copy to the frequency textbox for you. Then it works the same as the simple version, just for a single string.
Any questions, feel free to ask.
Note: requires flash player 8 to use (get it here