Here is a paper which has a complete recalculation of garnet formula from EPMA data:
A. Locock, Computers & Geosciences, 2008, v 34, p 1734-1780: "An Excel spreadsheet to recast analyses of garnet into end-member components, and a synopsis of the crystal chemistry of natural silicate garnets."
If you can't locate a copy, let me know and I'll send you the pdf (with link to the excel spreadsheet).
John F