if the syringe is marked from 1-100 then each tick is 1 iu (for insulin). if you're saying that 10mcg = 1iu and you want is 20mcg, then you want to fill the syringe only 2 ticks.
if you filled it 10 iu's, or 10 ticks, you'd be getting 10*10mcg or 100mcg.
are you adding bac water? if you are/did, then just add more to make it easier to measure out.