What determines the OAL for my reloads for my new HK USP? I used to just use the "book" length for it, but I think personally if it fits in the clip and functions, any length is fine. I've been reloading for 20+ years and am just getting an autoloader for the 1st time. What do you think?

Your first plan will probably work just fine but I have had better results by using the barrel as a guage.I have never loaded for HK but this works for 1911s, CZ, Browning, Norinco, Walther, Glock, Beretta and S&W. I don't see why it won't work for yours.
Remove the barrel from your gun and hold it chamber end up. Drop a dummy round loaded to your length into the chamber, it should slide in until the base of the case is even with the highest part of the barrel. Adjust your OAL until the base of the case does match the top of the barrel and you have a custom fit (which is also max OAL.)
Once you have the ideal length you must make sure it fits into the mag and feeds reliably so make 5 dummy rounds and repeatedly cycle them through your action,try using the slide release as well as pulling back on the slide. If it feeds you are home free if not you will have to shorten your OAL.
The benefits I have found are greater accuracy, reliability and less leading so I use this approach with all of my semis.
Guy

I have loaded for several autos and have found that you have to watch 3 things: mag. length, throat length, and ejection port length. Too short can sometimes cause as much grief as too long. I usually load a couple of dummy rounds and check for function, then load a small batch and try them at the range. Anything less than 100% function isn't acceptable. And remember to keep notes. This really helps when you want to try some loads that you haven't used in a few years.
Hope this helps.
C.G.B.