Just some thoughts (based on experience)
1) TDC will only be valid when BOTH the inlet AND exhaust valves are closed and the piston is on the up/compression stroke. You can not do it off just the cylinder height as whilst the piston may technically be at TDC the crank can be in any of four positions (four stroke engine)
2) On my old Tipo 1.6 Brio (K reg) I seem to recall that for some reason Fiat chose to do TDC off No.4 cylinder and not No.1
3) Doing TDC with a dial gauge is not very reliable if done off one cylinder only. This is because piston displacement is minuscule at TDC for crank angle. The best way to do it is with TWO dial guages to get No1 on the compression stroke up towards TDC and then rotate the crank backwards slowly. You are looking to get No1 & No4 piston or No.2 & No.3 pistons dead level. The smallest error in crank angle will make huge changes in piston levels. With this levelling set then the crank is exactly 90 degrees before TDC. Now you need a protector to dial up +90 degrees
4) The cheats next most accurate method is to remove the battery, possible battery tray, or any other item that will allow you to get to the top of the bell housing. Here you will hopefully find a rectangular rubber bung (5cm by 3cm approx). Remove this bung to reveal an inspection / timing window. On the edge of the window casting you will find 3 notches. The left most (as viewed) is 0 deg./TDC, the middle is 5 deg. before TDC and the right most is 10 deg. before TDC. There may even be a 0 / 5 / 10 legend. If No.1 (or No4 if Fiat switched the reference point) is on the compression stoke at TDC then you will find a reciprocal drilled notch in the flywheel which you can accurately align with the 0 mark.
5) Most modern Fiat / OEM timing tool kits have both a crank TDC location setting tool/plate and valve/camshaft setting methods which take the errors out of setting both TDC and cam shaft positioning.