@Snake-Pepskin said:
The auto pens are a bit more tedious because
It still works by making a good guess at how much to dial + 20%. Press the button and the dial and plunger stop, but the dial does not go all the way to zero because no gas can escape without the needle installed. Then just back off the dial. Only a little pressure is left on the vial. Insert a needle and only a little gas blurts out.
You can see the status of the bubble by holding the pen upside down, above your head, while a light is shining up from below. You are looking up into the needle end of the cartridge. This way, the gas bubble reflects the bubble size quite obviously and you can dial and purge without losing much fluid at all.