I assume that it is a single-start lead screw ...
If you measure the pitch with a good ruler you will get it close enough to choose among one of the standards ... there aren't that many to choose from.
But at the end of the day, it probably comes down to pulses per distance. I strapped a USB camera to the Z stage, put a good ruler on the table, then moved it a few thousand pulses and measured the displacement. The camera allows a measurement to ~0.005" accuracy, so on a 30-40" travel you re certainly good enough.
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