Using the PET CPBF. Usually bottles would always foam up a little when filling, squirt beer when taking the bottle out of the filler and by the time the lid went on there would be a few cm of foam in the bottle. As a result my beer always came out a little undercarbed.
The 'secret' is very simple: Just fill very, very, very slowly. Purge bottle as usual, turn top handle to beer out and open, then very slowly turn that chrome lever, until only a trickle comes out. Fill bottle until it's full to the brim, just push the top handle down, leave the chrome lever where it is, take bottle out and quickly cap. No foam at all, and more beer in the bottle, so less (actually no) space for gas to escape once capped. Works with new caps. Takes about 30 to 40 seconds per bottle, at a guess.
Not sure why I hadn't worked that out earlier...