Is there a standard way I should calibrate a new pen?

I've seen videos showing how to load the pen and then insert a regular syringe into the cartridge stopper to deliver 10 units and see whether the syringe reaches the 10-unit mark. You can then use that syringe until switching fully to the pen. The syringe stays in the rubber stopper and the pen plunger simply pushes the test syringe plunger back. If the reading is off by a fixed amount you can note the difference and adjust future doses; if the error changes with the number of units delivered the pen should not be used. Pens like this cannot be adjusted internally.

All four methods you described are valid, though their accuracy depends on syringe size, so large differences between them are unexpected. Repeating the same method should give steady readings. I stopped using the flat-top retractable pens and now rely on other models that deliver consistent force without skipping. The cartridge seal can sometimes be installed upside down, which affects movement; the thick side should face the shaft and the thin side the liquid, just like in an insulin syringe.

When doing the reverse injections, were you using brand-new syringes each time or flushing them with water first? Air left in the needle can compress and throw off the measurement. Flushing first so a little water remains in the needle should give a truer reading of what the pen actually delivered. Not sure if that explains your results, but it is worth checking.

Mine is off by a steady amount, so for a 20-unit target I now set 24 units and that lands closest to the desired volume when checked with a 1 ml syringe. I get repeatable results with each method, but I settled on the readings from the 31-gauge tips since that matches how I will actually inject. I started with fresh syringes for every test but later reused them after the first fill and saw the same numbers each time. Lower-gauge needles seemed to deliver more accurately, yet I will still be using the higher-gauge ones in practice.