You are partially right. This is order of operations.
In this case, you are sending -.08 to the Round(2) filter.
BUT you are wrong that parenthesis are not allowed. You absolutely can do something like :
{{ (Volume |float / 100 + -0.8) | round(2) | abs }}
I can’t make out from your problem statement if this is the right way to structure your formula, but the point is you can change the order of operations with parens.