I've filled it above the head and usually lay them on their side, the only leaking I get is when the top of the silicon gets loose and flips inside the air hole when tightening. If you get this you can fix by using a different gasket or flipping the metal piece in it upside down and put it back using the bottom non worn out side as the new top. I also remove the drip tip and stick something in it (usually tweezers) while tightening to make sure it stays out of the air hole.
He's correct that, once filled and top screwed on with a good silicone cap properly placed and wicks covering the gaps it won't leak into the center, even if you lay it on its side.
The reason most of use fill to the bottom of the wicks is to stop leaking into the center WHILE FILLING. The top isn't holding the silicone cap in place while filling and it often gets pushed around by the juice bottle if you use that to fill. Maybe you can fill more if you hold the silicone cap and/or use a syringe. But you still have to take your finger off to put the top on. The top puts pressure on the silicone cap and that in turn kind of presses on the wicks. So everything is "tight".
Also, if your wicks are messed up or not filling the gaps properly it will leak into the center, but it would do that when on its side too no matter how far you fill it. Make sense? The opposite is also reported: people having problems with the silicone cap compressing the wicks too much. Haven't had that myself, but it seems to be common.
Remember: Flat side of the metal washer inside the silicone cap goes pointing down toward the wicks.
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