You need to do some math and figure out your mg/ml. Then just take less than a mg that would equal that dosage.
For example 100mg per 1ml, in order to get 50mg dose one would take .5ml.
You don't have to make it a specific concentration. I am assuming you are just trying to reduce PIP. Just draw 1ml of gear if you want 300mg and then add in some oil to make it less concentrated. Maybe that wasn't what you were asking though.