Determine pipe diameter with 2 known values

Ah, I see it. The MATCH isn't really happy with smaller rain water load, especially if its below the smallest number there.

Made a small change to the formula.

Formula:

=INDEX($G$8:$M$9,1,MIN(COUNT($G$8:$M$9),IFERROR(MATCH(B18,OFFSET($G$11:$M$11,MATCH($B$20,$F$12:$F$21,0),0),1),0)+1))

Thanks a lot for your help and quick response!!

Regards,
KJ