I found a very inexpensive balance scale at Harborfreight.com, and used that for a while.
Then, I learned that proseal cures in a catalyzed reaction. What that means is that if you mix any of the black goo in with the white goo, the mixture will eventually cure. The only difference is that the more more black goo you add, the faster the mix will cure. Higher temperatures speed curing too...
Once I had that straight, I simply used throwaway plastic spoons to scoop out the approximate ratio recommended on the containers.
400 or so hours and 4.5 years flying, and I've had no problems with the tanks..
KB