are you able to post some sample data?
This seems doable with a parameter and I could give it a go!
It depends on your view and level of detail, but if you are using a quick filter this approach will work.
I'm using basic Super Store sales example:
The original sum of sales is on bottom. the calculation is on top. here are some examples of the output:
for Paul.twbx 1.1 MB
That looks like it might do the trick. I need to prove the calcs, but they look good on first glance!!