I had a tough time mixing it also, so I went into the kitchen and 'borrowed' two plastic measuring spoons. One is 1 tablespoon, and the other is 1/4 teaspoon.
I use 2 tablespoons of bondo to 1/2 of the smaller teaspoon (because we didn't have a 1/8th teaspoon) of hardener, and mix it in the cap that comes on top of the bondo.
This makes enough to fill several small holes, and do some edge repair before it hardens. Any less would use such a small amount of hardner that I imagine it would be hard to get the mix right.