Where exactly to center pulley when adjusting timing??

[nissan240sxmn]

I know it might sound stupid but on what timong mark should you center the pulley when timing? AT TDC or at the 20 degree BTDC mark.

because this is how i do it i make sure 1st cylinder is at TDC which will make the pin to be at the TDC mark on pulley then to make sure the distributor is pointing at the 1st cylinder is this correct.

or should i center pin at 20btdc from the beginning then put the ditributor to point at 1st cylinder?

i know it's suppossed to be 20btdc somewhere.

[AZhitman]

240sx Timing