Question on opening for FB to cooker

[Bruster]

Is the measurement for the opening side to side or bottom of tank to the cut?

The half moon right now is a little bigger at 17.50

[Big T]

This is a little confusing for most guys (me too) and even harder for me to explain. You should check out Pete's throat calculator , it explains it a lot better. There's a link to the pit calculators at the top of the page.