Thanks for the backup there Troy
I've been thinking about this a little more, and another thing you can try is use a slip fitting with tensioners on both ends rather than the ball valve. What the slip tensioners allow you to do is loosen one end and slide the drain tube up and down to get the correct water level in the skimmer, then you tighten back down the nut and it's water tight again.
I think these are just about the same price as ball valves, but may be easier to adjust.
These slip fittings are normally used to repair pipe, but it should work for this application.
Hope that gives you some idea of what your options are... it looks like you're well on your way to being a DIY junkie...