I figured out why I couldn't upload the spreadsheet - and corrected it (they only allow 10,000 documents... I adapted it by making more columns so it would work...
http://spreadsheets.google.com/ccc?key=pJtFqy9H_vgzAo2LZ7cFObQ&hl=en
This document is fully changeable... unfortunately some things don't work as well on Google Docs as they do in Excel. No Dropdown boxes. Also I can't "lock" cells, they are wide open, so someone could screw it up... I have my own backup in case it gets corrupted.
Use the two letter expansion code (I used the typical codes) on the first page. It takes about 15 seconds to update the screen after you type in the code - be patient.
Look down the sheet for the correct booster composition... eventually I hope to adapt it to show the booster in one place - rather than scrolling to the correct one. At the bottom of the sheet is where the "odd" types show up (Time Spiral, Planar Chaos, and Alliances)
If you want a completely random booster for 15 card rare, 3 uncommon, and 11 commons, you can select the second type... They won't work with the 8 card booster types since they don't contain rare cards. It won't bring in "special" cards from TS and PC.
If the top booster composition lists multiple Random cards (1222###########), you need to use that booster generator. I use that for the expansions with so many non-single color cards where it makes more sense.
Enjoy!