Anyone know where I can get one?
Seriously, my buddies had one that had the CFM for any and all kinds of duct. I was like a little slide-rule thing and you just picked your numbers and it told you what CFM for what size duct or vice-versa.
I've been wanting one forever...mainly so I could try to commit some duct sizes and their respective CFM to memory. I just can't really seem to fing one though.