Ive been plumbing for ten years and in one hit you’ve just showed me several tips that will improve my work rate and skills ten fold , thank you.Clever idea using two hole saw blades on one arbor so the smaller one centers the drill for the bigger one! Another good idea to drill a hole in a piece of plywood using the required holesaw, then clamp it over the area where you want to drill the hole to stop the drill moving around. I've used this trick several times. This is really good when you don't want the larger hole exactly centered over the smaller one, or if you want to make an oval hole using the same size holesaw by joining 2 holes together. This also works for diamond tile drills, and in fact I've made up a small plywood template which I keep with my tile drills.