I want to make my own nose pieces. Is the hole diameter critical?

Hmmm...I use a Davidson Seating Depth checker too. I also have a Sinclair Bullet Comparator which looks like a large hex nut that screws onto a gigantic bolt.

I don't think the nosepiece diameter matters as long as that is the sole reference point you use when seating bullets. If you were to measure one cartridge with two different nosepieces each having different diameters, you'll get two different Combined Overall Lengths (COL). Assuming you already have an accurate load developed for a rifle(s), you could use your fabricated nosepiece to measure the COL of that load(s) and document your new COL for that load using that particular nosepiece and bullet manufacturer and weight. Obviously, if you change from one bullet manufacturer to another, the COL will change once again since bullet manufacturers don't necessarily position their ogives in the same place on their bullets and their profiles are slightly different too.

like Sep said.. also i use a stony 30 cal comparator to measure the length to the shoulder on smaller neck cases, checking for set back after neck or fl resizing.
Dave