you calculate all hits from the same weapons at once. so:
I shoot you with my Tac squad.
5 hits from bolters
1 hit from combi-melta
1 hit from plasma
you delegate hits from melta first (i would at least)
then from the plasma
then from the bolters
that way if the bolters take out a few, then the lower power saves i'd use my 3+ to save
some say it is cheating, i say it is the rules.