Put the hole about 1/2" above the bottom skirt weld. I think that's somewhere between 3 and 4" from the bottom. Also, look out for the drains in the skirt. If you are firing the keggle via propane a lot of heat will come through there and really heat up your valve.
...and try to get it 90 degrees from the handles in the skirt. Best bet is to stretch a piece of string around the skirt, grab the string at the edge of the handle holes, fold the string in half, then stretch that half of it around from a hole to find the center point.
I forgot about my keggle playlist. This is a pretty good guide. All of these videos are from HBT members.