make an autofill based on the number that shows another field.

I need to make an autofill based on the number obtain from the substrack of 2 fields.

Ex.
1.-
Field "A" quintitie assure 100,000
Field "B" % charge              .4%
Field "C" total charged           400
Field "D" comission paid      0%

2.-
Field "A" quintitie assure 100,000
Field "B" % charge              .6%
Field "C" total charged           600
Field "D" commision paid    .1%

So field E I needed to be
  
Field "A" times  x points (If  in field B-D .4%= 3 points, If  .5%=5 point. If .6%= 7 points , all with commission considered) 

1.- 100,000 X  3 (.4%- .0%= .4%) = 300,000
2.- 100,000 x   5 (.6%-.1% = .5%) = 500,000