[Figured it out, if anyone is interested]
Something along the lines of:
=SUM(Iif(Fields!A.Value=0,0,
Fields!A.Value))
/
Iif(Fields!A.Value=0,1,
SUM(Iif(Fields!A.Value=0,0,
Fields!B.Value)))
So this assumes (in my case) column A is the one that typically falls behind in updates. I gauge the presence of a number in that field for each row to calculate both numerator and denominator.
Worked fine for me. Maybe it will help someone else...