A good, meaning not a HF dial calliper is suppose to be accurate to a thousandth of a inch, a micrometer needed for a piston of the size you need provided it is set properly is suppose to be I think to a ten thousandth of a inch
A lot depends on whose taking the measurements and is the mic or caliber accurate
Im no machinist by any stretch however I was trained in basic machine shop and did some basic lathe and milling work
Dont listen to some guy who tells you that using a dial caliber, you might as well be using a yardstick, he's a guy looking to build himself up at someones else's expense
Yes you should use a mic but again if you dont check that mic, if you have little or no experience using one, whats the sense
Talk to real machinists, let them explain the do's and the don'ts. Why something will work and why it wont work
You also have to take into consideration a mag drill will move, it depends on not only the strength of the magnet but how thick material it's attaching to. A engine block would be certainly thick enough but putting too much muscle into the handle on that press, do not think it cant move.......The other issue is the amount of play in the press's ways....It's a drill attached to a magnet.....Not all are made that way but yours is. The tool was designed to drill holes.
No expert here but just pointing out what I feel issues might be