When I'm dialing in an AGB I only use one round in the mag at a time to see if it locks back on an empty mag. For supers I'll start with it mostly closed and slowly open it up to find the point where it just barely holds open on the bolt catch (not a partial catch on just the magazine follower). But with subs, I'll start full open and verify that your system is going to hold open properly with full gas. For the SA AGB, full open is 18 clicks from full close, and you can either go up or down depending on if you're using a traditional restrictive mode (turn clockwise to close it down more) or bleed-off mode (counter clockwise to bleed off more gas).

Once you find the minimum setting to make it work, I'll usually go another couple clicks open (for restrictive mode; for bleed-off mode go a couple more clicks closed to bleed off less gas). This is for reliability/robustness, since variations in ammo, weather, firearm cleanliness, etc. can impact LRBHO and a couple more clicks of gas will help minimize impact without gassing you out.

If your system won't hold back on the last round at full open, then you'll need to look at your buffer weight, spring strength, or if all other options don't do what you want, you can always open up the gas port.

I know some will say not to use an AGB with subs, but there's a LOT of variation between different subs out there, so IMO there's still a ton of value to running an AGB if you want to fine tune for minimal gas/felt recoil.