Other than remembering that the 30 degree rule is relative to the ground, not the belt, you have it. If your belt is at an angle, that will affect the 30 degrees some.
I find when drawing, if there is a significant angle, like right at 30 degrees, the draw is more of a side motion with your shoulder and arm. The more vertical the draw, the more the draw is done with your elbow, moving your hand upwards. You may want to try holding a holster at different angles and different positions with your left hand until you decide which is best for you. Then make your holster accordingly. You may like 15 or 20 degrees better than say, 10 or 30. There is no right or wrong there, just your preference. Another factor to consider is how far forward it will be. In other words how far you want to twist when drawing and rte-holstering. Remember the holsters must be "located one on each side of the belly button and separated by at least the width of two fists at the belt."
I look forward to seeing what you end up with.