@Dodi do 900 Are you sure that you want us to roll 5d6x3? I just rolled and got 63... that seems a lot.
@Dodi do 900
I was going to say do it like D&D stats. So do something like 1D8+1D4 (Usually this is a number rather than a roll).
If you roll anything lower than 5 on the D8 treat it as a 5
But it is Dependant on what your health of enemies is going to be like.