However there are pieces that I dip not all the way down and leave a little of the clay exposed. On those pieces I don't bother wax resist. Is there some trick to getting a perfectly level dip? I was thinking about buying something like this: http://amzn.com/B000BQR7TU
Why not measure how far down from the top you want the glaze to be and only fill whatever you use as a glaze bucket to that depth - dip your pot (upside down) until it sits on the bottom - assuming your table/work bench is level the glaze line will be level.
Some experimentation will be required to allow for displacement of the glaze as you dip your pot.
Using a spirit level is going to take too long, you will end up with too much glaze on the pot as it sits soaking while you study the bubble.