Ok so I have posted this info on my other thread 1275 Rebore.
Using the DTI Gauge I found TDC and then set the gauge on the block face and zero'd it. Then gently releasing the magnet I span it around over the piston crown and re-engaged the magnet. Doing this a few times and on 3 of the 4 pistons I found the piston to deck height to be 15 - 15.5 thou.
This in not an accurate method to measure TDC.
If you are going to use a dial gauge, then mount it so the plunger is over the gudgeon pin, not off to the side of it like this ^. With the normal piston to bore clearance and the rock you'll have on a cold piston, that could be anything up to 0.010" off.
Also, if using a dial gauge, always measure with the piston rising, not falling.
A more accurate method again is to use a Piston Stop.