It would be nice if you had either separate fuel ingredients or a jug of FAI fuel and a jug of 40 or 50% to custom mix small batches in your wife's nice Pyrex mixing cups.
If you want, you can arbitrarily say what the highest % of nitro is that you want to run...then go from there. What ever you're comfortable with.
If you fail at finding the right tune for 10 or 15% nitro...you'll always have other engines to burn that fuel with.
Very few Nelson engine owners [that I know of] ran them for the same reason that you are running your's. They can give generalized advice that gives you a ball park idea, but Dave Shadel is the guy I'd trust most for prop suggestions once you are able to furnish him with your engine's timing info.