If you want to show to customers that you support MCS8&9 you should be able to allocate 5 subslots per cell. Of course if you don’t want to waste resources you should use some dimensioning, but the Erlang formula is not usable in this case. We are doing the same as you wrote. Per each radio TS CS – 1 subslot; then for PS at least 5 subslots per cell regardless of the number of PS radio TS. Then if utilisation goes over 80% or you have blocking we assign more TS to that site.