I have seen a really good method for this, aimed at phramacys.
the basic principle is you mix the total ammount of all the ingrediants you mean to use in together.
i.e. 50 caps at 100mg / 500mg of a substance
mix 100mg x 50 of A ( so 5g)
500mg x 50 of B ( so 25g)
I dont rembmber the exact method, but it focused on starting with your 5g of the lower dosed substance, and gradualy mixing in more and more of the other ingrediant using a pestle and mortar. its important to ensure a even distribution of the two, as other wise you could end up with 'hot spots' with too much of the more potent substance.
cant find the original with a quick search, but its here somewhere I think!