It just needs to be above the water lol.
On second thought I just remembered when I had set my first tank up ages ago with this fuel line like this as a vent. On a hot summer day I had my fuel line up high and got fuel dribbling out of the vent line. It was not until my first Boxer bike purchase and I saw how they vented it that I don't use that hose as a vent any more. Just finessed my fuel cap to breath and plugged that port off for ever. Works perfectly for me...
Their tank had that port blocked off plugged. There was only one line coming out of my tank the fuel line that was it. Only thing was that later I discovered my cap needed modded to breath and that the fuel filter the tank came with ''Goped suppler'' had a very small restrictive fuel filter.
I gutted the fuel filter out and used it to attach my hose. Then put a nice inline filter down the line. It has been pleasant ever since.