The context here is an endurance setup, with a fit optimised for the hoods/drops. Clip-on aerobars are being fitted to give an additional hand position especially into sustained headwinds; this very different position relieves pressure and stress on the hands and wrists. We (it's me helping a friend - I've done my own by trial and error) don't want to compromise the rest of the bike fit, which was done professionally, and which will be applicable for the majority of the time.

I've seen some advice about height ([example](https://www.visiontechusa.com/en/news/how-set-aerobars-perfect-tt-position), but that's in a TT or tri context, aero above all else, to the extent that they recommend a reduced crank length so the knees don't come up as high. Even then they don't say much about the reach on the aerobars (when that's adjustable without changing the stem). I know from my own recent adjustments putting mine out too far that it can be quite important, as my thighs contact my ribs.

For other aspects of DIY bike fitting there are plenty of more-or-less accepted approaches, but what about for aerobars, whether that's for endurance or a TT novice? I know a bit about the principles (I discuss them in [an answer](https://bicycles.stackexchange.com/a/83944/7309) along with some more details on my setup) but wonder how to match those principles to a user. We don't have a turbo trainer available, but have good roads for test-riding, and I'd be interested to hear if trainers added something significant.