At the moment I only can think of the following approach.
Product A - raw-material = 1 unit (this is what I purchase from supplier)
Product B - variable raw-material = 500 units
with Product A = 1 unit as BOM
Product C - variable = 50, 100, 500
with Product B = 50 unit as BOM
with Product B = 100 unit as BOM
with Product B = 500 unit as BOM
Doing it this way, will cause more products needed to be created for the calculation.
I think there must a userfriendly solution to this.
There should be an option to define a sub unit count to use in situations like mine which happens on regular base to others for sure.