I purchased the FastTech one and it reads exactly .2 ohms high (as checked against two VOM's). I didn't return it, I just use it as is. If my coil reads 2.2 then I know it is actually 2.0.
Hey that's exactly how mine reads! Usually 10% high. I ordered another one from a US vendor and it also reads about 0.2 ohms high.
Doesn't seem to be a way to calibrate these.
I also got an in-line battery checker that screws right into the mod. But I found that a kicked mechanical will not show any battery voltage. Busardo mentioned that in one of his videos, so I wasn't surprised.