Roll-up or formula in grouping items

I’ve designed a custom field, with calculated values. My problem is that I need grouping items to use that formula, in order to summarize data. I’m not able to do that, can anyone tell me how?
It would also be nice to have a kind of roll-up for those grouping items, is that possible?

(By the way, I’m using Primavera 5.0)


Thanks
www.khorramirad.com [1]
info@khorramirad.com [2]