Dead simple lathe tool height gauge

[Toolmaker51]

I'm afraid my method wasn't very exciting or innovative. I already had, through much fiddling, shimmed a facing tool to be perfectly on center and leave no trace of a center pip when cutting a variety of materials, even softish aluminum. I merely stuck the height gauge on the top of the compound and measured the height of this tool, then duplicated that height on the gauge I made.
Had I not had that already-centered lathe tool available, I would probably have used a method very similar to what TM51 described...If you use TM51's method, be sure you have a truly cylindrical rod and hold it in a collet or, lacking collets, use a 4jaw and take the time to get it running true.

Marv's establishment of tool height by physical test is accurate and effective; pointing out that he shimmed until the pip was eliminated. His, as some lathes haven't a machined top crosslide; so the compound is a natural.
The alternate is turn a diameter and measure; collets or 4 jaw wouldn't be a requirement, just a skim cut with decent finish Size is beneficial, but not crucial at all.
For cast surfaces there is a solution too. Construct a base having three contact points with corresponding areas on the crosslide, close enough to reach toolpost.
For example; might be 3 locked setscrews protruding from the slide into center drilled pockets of the base.