Calculated field in a Rollup field

Hi. Could you help me please? I need to use simple calculated field in my Rollup field to:

I read that Rollup field can use common calculated fields, but there's no these fields in Rollup filter. What's wrong? These calculated fields are simple and not use another calculated fields, for example (fields in quotedetail):

If "isproductoverridden" = 0ThenValue.MyCalculated.Field = product.MyNumericFieldElseIf "isproductoverridden" = 1ThenValue.MyCalculated.Field = mycustomentity.MyNumericField

Why I can't use this calculated field for Rollup field in quote?

P.S. Maybe there's some another way to count it, like use workflow or something like that - but I checked it, quotedetail can't be edit in workflow (don't know why). Any another ways?P.P.S. I tried to not use this field in Rollup, just create a business-rule for a new common field that take a value, and this field can be used by Rollup, But there's another problem - user must save record to make changes of business-rule to work. Maybe there's a way to automatically save changes for business-rule without opening form? (but only for this form - not for all CRM)