Chainsaw Repair

The trick is being able to have a clear view of when the piston opens or closes the ports. Like Al said it is not hard to be off a weee bit one way or another.
I struggle with the upper transfers "peeking" through the spark plug hole at an angle  Then there is the ex., by using a flashlight through the sparkplug hole, do you measure as soon as the first bit of light slips out & onto the floor of the outlet or when you see it exposed at the crown of the piston If you go with the first sliver method you have to accept that the chamfer on the port is letting the light sneak out.

Steve