Place a mark on the crankcase in a spot that's easy to see. Move the piston to TDC (roughly), now move the crank until the piston JUST starts to move down. Now make a mark on the crank even with the mark on the case. Now move the crank the other way and do the same thing. Find the mid point between the two marks and you have as true a TDC as your going to get. If you have a dial indicator you can get a little better reading.