Just looking through some un-answered posts and came across this one!
So although we have the Tableau generated Measure Names as a dimension, it is not a 'real' dimension (in that it doesn't have the same behaviour and flexibility as a dimension in the data). As such we are unable to use it as a partition in a table calculation.
however it looks here like what you want is the %age of Product Count (as a %age of total products) which we can do. If you can let me know what you are trying to get to I'm happy to take a look.