Probably, the original poster already got more than he bargained for, but for the records, a few more comments:
All FFT based structure factor programs require that the sampling rates along each axis be even.
I don't think this is true, unless I'm misunderstanding what "sampling rate" means. The FFT in phenix is based on FFTPACK (written in the 80s) which works for any number of grid points. FFTw also supports arbitrary gridding.
They may have other required factors depending on the space group, but they will be happy to inform you if you make a choice it doesn't like. They are also more efficient when the prime factors of the sampling rates are small numbers. Try to stick with multiples of 2,3, and 5 if possible.
That's good advice. FFTPACK is fastest for transform sizes that are multiples of 2,3,4 and 5. The map calculation algorithms in phenix automatically take this into account.
All FFT programs will fail if you sample your map courser than twice that frequency, as SFALL did for you.
This may be true for SFALL, but not for the FFT algorithms in the phenix libraries ($PHENIX/cctbx/include/cctbx/maptbx/structure_factors.h). The critical reference is: David A. Langs (2002), J. Appl. Cryst. 35, 505. As short as it is, this was an incredibly important paper. In retrospect, it is amazing that it took so long for someone to discover the trick. Ralf