Doesn't work that way, you need the slip yoke that will be supplied and the driveline shop will determine the difference between the measurement you have and the dimensions of the slip yoke being used.
You don't make any measurement calculations, you bring that 50.5 measurement and they'll determine the length the shaft needs to be with the parts chosen.
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Actually it's best IF you have the slip yoke you're going to use to measure it be BOTTOMED out in the trans and let them have that measurment and make the shaft from that, the ground hub surface and slip length are different on many yokes and suspension type all play a role in why that slip is that long and what needs to be in the trans and how much needs to be out.
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