John,
Is the X direct drive or geared (belt or gears)? You are off by a factor of 2.5.
Steps per mm = total steps per rotation / mm traveled per rotation
The terms "per rotation" cancel out and leave you total steps/mm traveled. You should be able to find the mm pitch by setting up a dial indicator and measuring how far the crossfeed travels with one revolution of the leadscrew.
The simplest thing might be to look at your current configuration and see what the current step factor is set for the X axis and multiply it by 2.5.
Alan
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