Leave the shaft uncoupled the entire time.
Imagine crosshairs on a rifle scope
site looks like 4 quarters, top, bottom and two sides.
Mark the top, measure down, underneath, and each side, you will notice areas that it's easy to get the feeler gauge in, other areas it's harder, if you use the gauge and slide it along the transmission flange to prop shaft flange seam, you'll find the area that is open.
Be sure your shaft is properly supported if the prop shaft "hangs" unsupported, your measurements will be off.
The front mounts, are your best adjusters, the rear minimal.
Front of the engine moving down moves the rear up , and vice versa.
It's right when you can't get the gauge in any where.
Rotate shaft or trans coupler and recheck in 90 degree increments
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SV Cloud Duster