I have a PSA 10.5 inch 5.56mm upper.  It runs reliably except that it often won't lock the bolt back on the last round.  The ejection pattern is only about 2-3 feet from the gun.  How do I diagnose over/undergassed and mitigate with the correct buffer weight?  Or are there other ways to correct it?

Try a lighter buffer.
If that doesn't work, try replacing the spring and o-ring on the extractor.

Ok, going to edit this… Note to self, don’t reply before first cup of coffee.

What is your setup other than PSA 10.5" upper?  PSA is typically overgassed.

There could be a multitude of reasons this is happening.  Some are easy to fix, others might warrant an upper replacement.
1. Your BCG might be short stroking (not going back far enough).  
2. Could be that your BCG is cycling too fast.  
3. Could be a magazine issue.  
4. Could be ammo.
5. Could be a misaligned gas block.
6. Gas port too small
7. Etc.

I would start with the easiest.  Mags.... is the issues exhibited on the same mag, or on different mags?  if different, are they older, same brand, etc.?

Then I would see if its an ammo thing.  Under powered ammo vs. mil-spec ammo (think commercial 223 vs 5.56 like m855 or m193)

From there, I would try and video the issue to understand what the BCG is really doing.  BCG too fast or too slow? (springs and buffers can help fix this).

If none of that works, then its probably time for a replacement barrel or upper.

All good advice before you go down the rabbit hole.

Had a similar issue with a PSA upper although 18 inch barrel. Would fail to lock back on the last round unless the bolt was freshly cleaned and lubed. Would start failing after a couple of mags. I swapped mags,lowers, changed BCG's, gas block and tube, everything I could change chasing it. Finally changed the barrel and fixed it. Maybe something in the barrel was out of spec? Good luck with yours.

For the record, my PSA 10.5" barrel had a GP diameter of 0.084" ( ) so I highly doubt it's undergassed.

What buffer weight currently?

Action spring?

How is your extractor?

What ammo you using?

Try American made 5.56 if not already done so.

I had a 10.5 that did this with Pmags but not USGI aluminum ones. In my case my bolt was too fast and a heavier buffer fixed this.

YMMV

Over gassed sometimes looks like under gassed if the bolt outruns the magazine feed rate.  Most commercial guns err on the over gassed side.

Check to see if the gas block is loose.

Correct me if I’m wrong, but wouldn’t it be a heavier buffer? If it is not locking to the rear, that means it is moving too fast. So a lighter buffer would speed the bolt up making the issue worse.

Or am I getting confused here?

I would expect the ejection to be more forward and/or further if it were cycling  too fast.

If there's too much inertial mass (heavy buffer) an under-gassed system won't have enough pressure to get the bolt to the latch.
Brass ejection is a big help at which way the system is leaning.

Sounds like a good suppressor host.

This sounds very overgassed. Sprinco Blue and H3 buffer or switch to vltor a5 and run a5h2,3, or maybe 4 and Green sprinco. Also make sure your ejector spring is sound. Even overgassed guns can run like a sewing machine if you have a stout ejector spring.

If it’s over gassed just get a BRT gas tube and fix the problem at the root, rather than tinkering with buffers and springs. It looks expensive, but is cheaper than some of the alternatives.

But I wouldn’t think an overgassed rifle would have weak ejection like you describe. On the other hand, PSA.

I installed the BRT gas tubes on my two PSA 10.5” and a Daniel Defense 10.3” CHF barrels because  I know these barrels are horrible gassed. No issues with the barrels suppressed or unsuppressed; no more gas in the face. BRT gas tube is a needed feature on overgassed barrels

Especially if you use a suppressor.