A power beyond port has pressure 100% of the time. I am not familiar with New Holland tractors but I can safely say that you should not hook it up to the power beyond. That is just to clarify since you said the port next to the power beyond. On our John Deere tractors we always had a P next to the power beyond port. If you are hooking up to a port right next to the "P" then you would not be doing a good thing.

However, we also had an R next to the P. The R was labeling the return port. If that's what you meant, that should work well.

If you plumb the return into your selective control valve that's really not the end of the world. Some planters have vacuum motors that spin down gently when hydraulic flow locks up because they have a bypass valve on the motor. Some do not. If your planter does not and if you hook up to the return on your selective control valve, you really need to make sure that you shut off by going to the float position rather than just going to the off position on your selective control valve. If you plumb in to a return port that's always open, this issue is eliminated.