Reconstitution is where most people get tripped up. You have a vial of a given mg strength, you add some volume of bacteriostatic water, and you need to know how many units on an insulin syringe equal your target dose.
The math is short. Concentration = vial mg / BAC water mL. Units on a U-100 syringe = (target dose mg / concentration) * 100. A 5 mg vial with 2 mL of water is 2.5 mg/mL, so a 0.25 mg dose is 10 units.
Two things are worth writing down every single time: the exact volume of water you added, and the date you reconstituted. Everything else can be recomputed from those two numbers.
I stopped doing this on paper and use the reconstitution calculator instead. The same app logs injections with injection site rotation, keeps titration schedules, draws half-life curves, and tracks vial inventory with stock, expiry and batch. It runs in the browser with data stored locally on the device, and there is an iPhone app.