Can I mix my own polishing powder and do you have a suggested formula?

Many experienced craftsman mix their own powders. All that is required are two ingredients. Aluminum oxide and oxalic acid. A good starting formulation would be 1 part of oxalic acid to three parts of aluminum oxide. Depending on what type of marble you are polishing it may be necessary to adjust the formula, either increasing or decreasing the oxalic acid percentage. If you are relatively new to marble polishing I would not suggest mixing your own powders. Stick to the premixed powders.
TIP For a really good polishing mixture try using tin oxide instead of aluminum oxide. Tin oxide is more expensive but may prove beneficial for certain marbles.