On a 223 AR with a single feed adaptor in the mag, is 2.39 OAL too long? Am wanting to load 69 and 75 grain bullets, but being used to 2.25 OAL for mag feed, they look very long.

IF the bullet isn't hitting the rifling, you should be good to go. I've shot some 75gr Amax in mine loading ss, no adapter. I forget what the OAL was but way too long for the mag.

OAL gauge read 2.39, from a 1st time user. Do I back-off from 2.39, if so how much?

I run Sierra 80 grain in my 223. The COL is 2.550".

This puts the Ogive .010" off the lands, in my chamber.

You need to measure your chamber to know were the lands are, if you are running anything longer than 2.260" that is max mag length.

Thanks for the input Fred and Cuda, now there is a baseline to work from.

68 and 69 grain bullets can be loaded to magazine length at 2.250-2.260". 75 grain Hornady BTHP's as well as Sierra 77 BTHP Match are designed to be loaded to magazine length. Hornady's 75 grain A-Max is too long to load to mag length and are for loading one at a time into the chamber.
My WOA National Match upper with 80 grain Sierra Matchkings to touch the lands the oal is 2.472" and they need to have .005-.015" jump to the lands. Just for info Sierra 77 gr. BTHP Match to touch the lands in the WOA National Match rifle the oal to touch the lands was 2.316" Your rifles chamber and throat may be shorter or longer. There is no useful need to load 68, 69, or 77 BTHP's to longer than magazine length at 2.250"-2.260". You'll gain little or no accuracy trying to seat them longer.