The position tolerance with the MMC symbol after the number is equal to the stated tolerance (which you know how to do) plus what is called the "bonus" tolerance. The bonus tolerance changes from part to part, and is dependent on the actual size of that feature. Bonus tolerance is the difference between actual size and the MMC size. So for a hole, the total position tolerance is: stated tolerance + (actual size - MMC). In other words, the bigger the hole, the more out of position it may be.Now, when you place the MMC symbol on a datum, that adds even more variation on your position. If the datum feature changes size, that amount of change would be factored back into the position tolerance. MMC on a datum gets very confusing, so make sure you first understand the effect of MMC when shown after the number.

Related:

• GD&T Discussion Forum, Dimensioning and Tolerancing
• True Position GD&T Tolerance Calculator
• Spherical True Position GD&T Tolerance Calculator