I inject fentanyl patches and what i do is i usually buy the 25 microgram patch and shoot half of it and save half. if i can get stronger i usually try to get the 100 microgram. I cut the full 100 microgram patch down in to a few decent sized pieces for my shots. The fent is distributed evenly in the patch so all the pieces should contain around the same amount. I chuck a piece in and prepare it, shoot it and if i need more i just do another piece. Thats a fairly safe way to go well as safe as you can be with fent, you can always shoot more if the piece wasnt big enough for you.
Edit: Just read that you have injectable solution 10 micrograms aint much just shoot the whole thing or half, if you want to be extra safe go less if you dont feel a good enough buzz then shoot a little bit more and see how you feel.