You do NOT want the tool stopping the piston "at" tdc and you do not in fact want it stopping where the crank, "arc" starts to flatten out. Rather you want the tool to stop the piston "down in" a ways, distance not that important, and then crank it around THE OPPOSITE DIRECTION so that the thing is stopped again at the same depth.
You mark the stop point in each direction on the crank, temporarily, and then measure halfway in between, and that point is TDC. If the original mark is accurate, that is where it will be.
I did not even do mine scientifically, but it has always worked faithfully. I built this one for SB Mopars in about 1972 or 73 and still have it.
I simply ran a tap through a gutten spark plug shell and threaded a bolt in there
One caveat....many "bought" ones have a poor way to lock the plunger. I have advocated adding a lock nut on the back side to rigidly lock them. You do not want them to wiggle
For me, finding the dividing point between the two resulting temporary marks is easy. I just use dividers. If you clean up the dampener, and lay down some white paper or even masking tape, you can "prick" the material with your dividers and you might have to go back/ forth a couple times. If you get the two marks either overlapping or approaching each other so that they are say, 1/8 apart, you can easily visually divide it from there and be pretty accurate.
If you ever studied plane geometry, you can make an arc with our dividers from each temporary, and at the top and bottom where the two arcs cross will give you the intersecting points for a straightedge across. This does not work well on some engines, with marrow balancers.
I used to use this a lot doing tuneups in the Navy. I found a few of the FE fords with slipped balancer rings, and often checked them before trying to reset the timing.
