you need to add the rect coordinates in gen monster example -
/coordinates
lets say you get 256384 256846 13564 <----- Ignore this one
so that leaves us with
256384 divided by 32 = 8012 256846 divided by 32 = 8026
so then 8012 and 8026 are the first set of coordinates you want .... if you want a central spawn (all mobs spawn in 1 place at a time)
then your full rect in Gen monster will be 8012 8026 8012 8026
If you want it to spawn in a larger area, then run diagnally and do /coordinates again and then repeat the above steps
Rect would then be 8012 8026 XXXX XXXX