How do you get a sum of all the related values for a column in a related table?

I have a [SqYd_Calc] table with columns for Length & Width. Then there are

virtual columns for SqFt & SqYd that auto-enter the converted values. These are working fine, and showing these related records In-Line with the Group Aggregate of SUM::SqYd is the exact value I need (77 in the image below).

However, I want to access this value for further calculations ([Price] = rate * SqYd). And although the Group Aggregate feature makes displaying this value very easy, I can’t figure out how to get the same value in the equation editor. I can only get the product of ALL SqYd rows. What formula is the Group Aggregate for SUM::SqYd using?