Don, Paul has some excellent ideas. Thanks Paul. But I would suggest that you verify that the mass of the moment arm in you balance is not so great that it actually throws you mix off.

As you move further out for your 2nd part of the mix, you are also adding that distance worth of mass to it from the arm itself.

Mixing small batches will have a larger percentage of error.

If the mass of the arm for the difference in distances is even 1/10 the mass of the smallest batch you plan on mixing, then you will have a 10% error.

If the arm is light enough, then you should have no problems. Don't forget to include the mass of the platform for the water cup in its 2nd position as this is part of the arm and part of the error.

Tom