How do you calculate the milligrams of CBD/THC in a tincture or oil?

"How many mgs would 15 drops of CBD oil be? "

The answer depends on the concentration of the CBD oil. You have not provided enough information to calculate the answer because the CBD concentration differs with different manufacturers and that was why labeling became an issue with cannabis. When you purchase cannabis products from a reputable manufacturer the products should have a label containing the amounts of each ingredient present in the Cannabis product.