Hi - I ordered a generic linear actuator set from amazon (

here) and it comes with an 8mm leadthread that has a 2mm pitch. When I used

this calculator (1:1 gear ratio, 1/4 step), it says 400 steps per mm.

After testing this out, I found it to be inflated, and did some back and forth testing and came to the conclusion that 325 was a much closer estimate (which would be a 2.4615mm pitch). I'm certain it's 2mm from crest to crest, so I'm really not sure where the accuracy is being lost.

I'm not 100% sure that 325 is giving me a perfect mm per rev, just much closer - I would like to get perfect accuracy but I was hoping a calculation would do that for me...is it common for these things to be off?