% of Total, no hierarchy

I'm trying to create a calculated measure showing % of Total Net Sales, something like Rank or TopCount but showing % Value. I tried "parent"  but since I do not have hierarchy in the dimension , it gave 100% for every dim value.

a - how to do it for a certain dimension (say Vendor) that does not Hierarchy, just has "[Vendor Name]"?

b - can this calculated Measure be done/ created on Net Sales but run for all/any dimension?