Well you KNOW how long it is correct ? Otherwise you will never be able to fill OUT the DROs for Yend. It will NOT hurt to trim the data if you overrun the length while probing. OR if you parse the screw data you can trim it to length there.
IF you are doing an index of the screw then probe it will still catch all the data for the total length of the screw reguardless of the pitch variations you just have to plan for it in the sub program.
Just a thought, (;-) TP