ProClarity MDX Editor - Create new measure using a subtotal

Question

I'm trying to create a % scale calculation.  For example, I have qty's that I want to get a % of the subtotal.
Small = 4 units
Medium = 8 units
Large = 8 units
Extra Large = 4 units
Subtotal = 24 units
I want to show that for Small there was 4 units ordered, which was 16.7% of the total.

I need to somehow create a measure that looks at a subtotal that is calculate for a previous level, & divides by the detail level.

Patrick