With a regular model this "reflex" just makes the lifting airfoil a bit less lift'y. It's not that critical at all as long as they are even but if you droop them so the upper surface is one long smooth curve then it's the same as putting in a lot of positive wing incidence and that would give you problems in that you would need a bucket load of down trim to make up for that. Just keep them so they are close to level near where the designer intended. It doesn't have to be dead nutz level with the stab but it should be close to level in your case apparently.

As for downthrust you just use what you need. There's no magic formula where the angle is keyed to any other angle.

Now as for the lack of down trim needed for inverted that IS odd. If it flies basically hands off and level both upright as well as inverted then that tells me that your CG is as far back as it should be or that the combination of reflex and downthrust has produced a magic mixture for you. The aileron reflex in combination with the airfoil is acting like a symetrical airfoil apparently. Just another example of SPAD magic I guess.