How do I determine the diametral pitch need in realign to the tpi/pitch of the screw?
I don't really understand what you are asking here, so I'll just make some general comments and hope that it will help.
Are you going to make this indicator "gear" from scratch? If so, Machinist's Handbooks, other books, on-line calculators and resources will give you the information and calculations you need. When I made mine, I made it to resemble a worm wheel with teeth that "fit" both radially and same angle as the helix of the threads on my lead screw, which was WAY OVERKILL for what is needed. I just did it that way because I was young and playing and cut the entire wheel, teeth and all, on my 15" SB lathe.
Don't think of this as a "gear" as much as just a simple indicator driver.
Pretty much any spur gear, with the correct number of teeth, that is thin enough so the teeth fit in the helix of the thread on your lead screw will work. You can easily face off a thicker gear if needed. The gear doesn't have to "properly" mesh with the screw.... it just needs to stay engaged and not bind up to keep proper timing. It's not transferring any power; just revolving an indicator plate with markings on it.
McMaster Carr site is great for downloading drawings (PDFs and models) for gears, etc.. Look up a gear with the correct number of teeth and check the drawing to see if you think the teeth will mate fairly well. Don't worry about thickness because like I said you can face it off if needed. Verify the tooth thickness and make sure the gear tooth will fit between the threads the lead screw.
If you have a 3D printer you can print out test gears for prototype.
Good luck,
Ted
