Hi Peter,
If you use a dial gauge in line with the plug hole, its reading will not be accurate!
As the piston moves down the end of the gauge will move across the piston, giving a false reading
This factor depends on the shape of the piston crown so cannot easilly be calculated
I measured the dial gauge travel concentric with the plug hole, also using a degree disc.
0.375 vertical distance on the piston was equal to 0.440 in line with the plughole!
This was with raised crown pistons, so different with flat tops??
I made a piston stop from an old plug and a long threaded bolt with rounded end,
I set the crank position with a degree disc and then set the stop at that
Once made keep the tool for further use only on that particular bike
HTH
John