Bob,

Do you just need the formula?

If you message me with your email, I can happily send you the formulas etc.

Jimb

If you have already got more than one thing that fits on the mandrel, you can make a plug that one of them fits over, and use that as a gauge. The thread is not super critical, it is the register diameter and face that matter.

Find 3 wires that fit into the thread form being compared. They need to be big enough that they sit on the sides of the thread form and above the major diameter of  the thread being checked.

I use whit grease to gold the wires in place. Often I use 3 drills for  the wires , choosing a size that fits into the thread. If the thread is quite course, then it can be difficult to get the 3 wires under the micrometer anvil. Then I use a piece of plate of some sort to span the pair of wires. I use it as a comparator instead of an exact thread pitch diameter. There are calculations for using wires to get the effective thread pitch diameter.

I made a mandrel that represents my S7 spindle thread and register diameter.  The 3 wire method was very effective. Being a 12 TPI thread the micrometer anvil does span the pair of wires. But I find it easier if I have a little plate over the pair of wires.

Neil

For some common lathes, like Myford and Boxford, you can get threaded adaptors on a MT2 or 3 thread for use to mount a chuck on a rotary table (that often have a MT hole). One such might be accurate enough if a good fit, though it could be a bit loose if a bit cheap.