Okay think about it this way. If you had 5mg and divide it in half (b/c 2ml of bac water), that's 2.5mg for every 1ml. If 1000mcg = 1 mg then you have 2500mcg in each 1ml of solution. So each 10th, unit, or "Tick" is 250mcg. So 2 units equals 500mcg when you mix with 2ml of bac water.
If you dose at 100mcg a better way to do it would be to add 5ml of bac water then each unit is 100mcg.