There is waaaaayyyyyyyy too much slop in the pump lock to use it to make a TDC mark. It's also possible that the pump is off any number of teeth so using it to determine TDC is utterly unreliable. Doing so would be just asking for a ruined engine with bent valves, broken lifters, bent rods, etc...
There are various ways to accurately determine TDC and they have been discussed quite a few times on this forum. I don't have the time or gumption right now to go into the details or find links.