Fillit right to the bottom of the filler insert every time, but only on the side stand. Don't set the MC upright even though you can get more gas in, if you do it will very quickly go right to the charcole canister and the MC won't run, the tank can't vent.
So, how do you do this ever time with ease. Don't put the nozzel into the tank to push up the rubber crap....
Take the pump nozzel in your right hand, with your left hand lift up the rubber venting crap far enough that pump will turn on (they actually make a plastic part to slip in to hold it there), I just hold it with my hand. Put just the tip of the nozzel into the tank, just through the filler insert. Turn it on and watch it fill, with a little practice you can fill it right to the bottom of the insert very quickly. Then 3 of 4 little shots of gas more as it all settles and you have a full tank. I can put 5.4 gal in my 5 gal tank with this method every time, very quickly.
And if you want a little more, stand the MC up and top it off, however you need to hit the road right away of it will run down into the charcole cannister.