Rob,
Here's the way I do it.. Maybe wrong but it's closer to what John said.
1/4. note = 96 b/m. That's three 1/8 notes equal to 32 b/m. 2X 1/8 = 1/4 note which you are playing in 4 beats per measure. This would put the tempo = 64 b/m for a 1/4 note and 96 b/m for a dotted 1/4 note. therefore, I think the tempo is, John says, 64 b/m. Hope this helps.