Finding True TDC

Do you have any suggestions etc. in respect of finding true TDC on a twin spark engine (Alfa Romeo 155 8 valve) with the cylinder head in position ? The problem I have is that there is a curvature in the piston crown directly under the spark plug position and when placing a dial guauge extension down the hole it is not stable and tends to run away due to the curvature it sits on. I've considered going the positive stop method but with the hole being offset could there be a chance of the piston tilting in the bore or is this not a concern ?

A fixed deadstop will work just fine.