Question about making a measure

Hello,

I am looking to make a measure where the equation is along the lines of:

x = a*b

y = x/sum(b)

then I need sum of Y's. I think that the confusing part comes in the form that I can't make a column because the sum(b) need to change with filters to give it a weighted value. Does the solution involve multiple measures and using the calculate function?

Just looking for a nudge in the right direction.

Thanks