What ratio of 5wt and 10wt oil equals 7.5wt?

Every source that I've read about mixing fork oil weights seems to imply that it's a linear relationship. For example, if you mix 50/50 10 and 20 weight, you'll get 15 weight. Based on that, I'd say a 50/50 mix would give you 7.5 weight. Keep in mind that it's not an exact science and there's a fair bit of variance in the actual viscosity of the same nominal fork oil weight from one manufacturer to another.

You can look at http://www.peterverdonedesigns.com/lowspeed.htm for a comparison of fork oil visocosities from various manufacturers.