Most kit control deflections are for moderate performance, or a flyable plane for an average pilot. (There are exceptions though). So I personally set my low rates to be what is in the manual, and high rates for whatever I can get, or what looks right, depending on the plane. But, then, I tend to like a fast response on my planes, and I tend to view the throws in the instructions as the bare minimum for the plane to get out of it's own way.

If the plane is intended by the manufacturer as a low wing trainer, and that's what you are using it for, then I'd set low rates as the recommended, and high rates for a about 125% as you said. If you really are worried it will be too much, try 90% and 110% and then change it after you've flown it.

If you have other guys around, I'd leave off the duel rates, set it as recommended, and get someone to test fly it, and maybe hand it to you on a buddy box, or up high. Then, if you don't handle it, they can take it back and you can tame it down on the ground. Relying on hitting a switch to get the plane under control often doesn't work unless you've been hitting switches for quite a while.

After you've flown the plane a bit, you can figure out how you like things set.