I cut most bill slots 10mm deep and that has worked OK for me. Lip angle is a complicated problem. There are very general guidelines but there are a lot of factors that influence how a bait will swim and you have to get them all working together for the best results. The simplest scenario is a shallow running (2-5 ft) bait with the line tie on the nose of the body that is ballasted to swim in a a straight ahead attitude parallel to the surface of the water. A general guideline is that a lip set at about 45 degrees will give you a good combination of depth and wiggle. A greater angle will dive shallower but will have more wiggle, and vice-versa. Like Dale pointed out, if the bait swims with a head-down attitude, that changes the "effective angle" of the lip as it moves through the water. It gets quite complicated since the lip shape, width, and length interact with the body's shape, size and ballast positioning to determine how the whole bait will perform. The best advice I can give is to model your bait on a commercial bait whose action you like. Most of them have been tested and tweaked extensively. Take your copy on the water, test it out, and then you will have a starting point from which you can then begin to experiment and tweak your bait to perform to your expectations.