I have my Ultegra triple FD adjusted perfectly except for a slight chain rub in the biggest ring and the smallest cog. It is not a cable adjustment. I believe I have to adjust the limit. In order to move the outer plate 0.5-1.0mm. away from the chain, I believe I have to turn the high limit screw CCW. How far can I un-loosen this screw? You can't back it out entirely, can you? I assume you can only adjust it so far and then it stops, right?

It sounds to me like you have to move your derailleur cage just a skosh to the right. That will require a cable tension increase and/or a limit screw adjustment. Either way, don't overdo it. You will only need about 1/8 turn. Too much and you'll find yourself throwing the chain to the outside.

Leave the shifter alone for a minute. Try to move the cage by just pulling on the bare shift cable. If that won't move the cage, back out your high limit screw about 1/8 turn. If that fixes it, (my guess) you're done. If it doesn't, try increasing your cable tension 1/8 turn.

This is great advice and you obviously know just what I mean. I have not tried moving the cage by pulling on the bare cable. What about that limit screw? It is not possible to completely unscrew it? It stops at some point, correct?

The limit screw will come all the way out if you screw it too much so don't do that. Honestly, if you need to adjust your limit screw at all, 1/8 turn is probably all you need.

Thank you