I don’t know of any way to roll up a percentage at universe level - you need to do it at report level. Have you searched here? I’m sure there was a really good explanation of why it doesn’t work. Or it might be on Dave’s blog.
Debbie
Edited to add - here’s what a search pulled up:
The second contains a link to Dave’s blog which explains how to deal with percentages and averages and how to use a new feature in R3 which might help you.