The formula is based on the assumption that the RSL (signalling) for 1 TRX is only 16kbps in average.
So you can locate 4 TRX into one Abis Timeslot (there is multiplexing and dynamic sharing of the 64kbps : each TRX could go up to 64kbps if other TRXs are not sending RSL at that time)
Amman, your case does not follow the assumption.
pix