Use simple proportion calculation to keep you in line. This assumes you're rehydrating with exactly 1mL of water, which is 100 units.
If 100 Units = 16iu, then:
Multiple the number of IU's you want to inject by 100, and then divide by 16.
For example, if you want to inject 2iu:
2 * 100 = 200; 200 / 16 = 12.5
You would draw to the 12.5 unit line on the syringe to inject 2iu.
For example, if you want to inject 5.5 IU:
5.5 * 100 = 550; 550 / 16 = 34.375
You would draw to approximately the 34.4 unit line on the syringe to inject 5.5iu
For the sake of comparison, let's say your vial only had 12iu of HGH and you mix that with 1mL (100 units) of water. If you wanted to inject 3.5iu of GH, here's the calculation:
3.5iu * 100units = 350; 350 / 12 = 29.2
You would draw to approximately the 29 unit line on the syringe to inject 3.5iu