What's best for your application I can't really know given the amount of requirements data above. The hardest and stiffest when cured seems to be Clear by itself. Have you tried that by itself? If that proves too brittle for other reason not mentioned, then add a small amount of tough to see if that makes it a bit more durable for you. 50/50 worked pretty well for my 1mm wall snapfit part requirements.
I like to think of it as mixing colors. Each material has specific properties that come through in the final mix in proportion to their quantities.
I'm building display-type functional prototypes now, and plan on trying varying mixes of black v2, clear v2 and tough v2. My first mix will be to add in an equal amount of black to some 50/50 I already have, get a baseline, and see how that feels. In this case I want black-ish plastic with mostly the properties I get with 50/50. My gut tells me it will be a bit less durable, a little harder, a slightly translucent dark blue-green gray in thin sections, but it will have a nice surface and look kinda cool with just a clear-coat sprayed on later.
My advice is to try to understand the essence of each resin on it's own, do thought experiments about how they'll behave when combined, then experiment and see if your ideas are correct. I think after a while you'll be able to just know how a mix will behave before you even mix it.