You want it to stop before TDC, make a mark, roll it the other way, mark.
True TDC is halfway between the two.
With the piston hang time around TDC, just marking where it is at the top isn't as accurate.
shim size doesnt matter too much.thats the point.you are using the valve as a piston stop actually....put in the shim and rotate the engine one way and make a mark on the fw where it stops....then rotate the engine in the opposite direction and mark the fw again(both times using the tdc mark thats on the bellhousing)
after you have a mark on the fw from the forward and backward rotation,take a measuring tape or equivalent and measure the distance between the 2 marks you made...do the math and split that measurement in half...thats your true tdc.......
actually the thicker the shim you use the harder it may be to get a measuring tape in the bellhousing hole....
to be honest ive always measured mine with the trans out or on a stand.....a useful tool is to cut a big chunk out of an old transmission that has the top 2 bolts and the tdc mark on it and bolt it to your engine.
so in summary: shim the cam on #1 cyl,rotate one way,mark fw,rotate opposite way,mark fw again..measure that distance and split it in half and make your" new" tdc mark