you are trying to measure an amount of weight using volume. this can only be done if you know the solution's concentration. since you mix the items, only you know the concentration
some basic info
a milliliter (mL) is 1 cubic centimeter of a liquid
a gram is defined as the weight of 1 cubic centimeter of distilled water at 4 degrees celsius at sea level. now we have established a reference point for weight
so 1mL of water is 1 gram of weight
a milligram (mg) is 1/1000 of a gram
a microgram (mcg) is 1/1000 of 1 miligram or 1/1000000 of a gram
lets say there was a vial with 2mg of a substance - thats 2000mcg. 2mL of water is added to it. now the concentration is 1mg per mL, or 1000mcg per mL.
a slin pin that is marked for 100 units is 1mL, so each unit is 1/100 of a mL. if 10 units of the liquid were measured at 1000mcg per mL, that would be 10/100 of an mL, which is 1/10 of an ML - 1/10 of 1000 mcg is 100 mcg
you must determine a concentration before you can measure the appropriate amount. if only 1mL were added to 2mg, 5 units would be 100mcg instead of 10 units