another way to pressurize the tank before flipping the prop is to pump a little more fuel into the tank with the vent line closed.... just be sure your fittings are tight on the tubing...when fueling leave the vent plug off until you see fuel coming out then plug the vent, and then pump a little more fuel into the tank, before you take the fueler nipple out of the tubing be sure to pinch it then plug it.

It's hard to say how many seconds of fueling to put in with the vent closed but with my electric pump it was only 2-3 seconds and with a hand pump it was about 6-7 extra pumps I believe.

When I do that method it only takes about 5-6 flips of the prop with WOT then idle and put the glow on and it usually starts right up.