When in doubt go with the smallest possible hole and offset it from the center as much as you can, this still allows you to have as much of the full drum sound as possible while still giving a port for mic'ing. Also, don't be afraid to tweak your tuning a bit. When I first started experimenting with ported heads years ago I found I needed to take my reso head up a half turn on each lug.
Also, if you are in a position where buying a new head won't be a financial burden then I would suggest leaving your current head in-tact and just buying a pre-ported head. All the big manufacturers make them, Remo, Evans, Aquarian.