7I96 Encoder Spindle tuning

With the 5K potentiometer attached what is the voltage across the potentiometer?
(from P3 to P1)

Be careful when measuring as these connections are not isolated from the line voltage

6.3v no matter the speed

So a 6V isolated supply should work

Note that isolation is key here, a non-isolated suppy
will cause damage

Where would I connect that to? The 7i96 or Spinx1?

SPINX1

+6V to SPINX1 SPIN+
6V common to SPINX1 SPIN- and KBEC P1
SPINX1 SPINOUT to KBEC P2

Isolation is key, make sure there in no connection
between the 6V supply and ground (with an ohmmeter)

So you are saying to rewire as in the attached diagram? The incoming port on the SPINX1 labels that as +5v. Would 1 V make that much of a difference?
 

I assume you are referring to this:

"The output of the control is a linear function of potentiometer rotation. The KBIC® can also be operated in a voltage following mode by supplying an isolated analog signal (0-7 VDC) to the input terminals P2 (+) and F-."

If that is the case, should I use +7 V or is that too much?, and I need to connect SP- to F -?.

No, the 6V supply connects to SP+ and SP-

+6V --> SP+
-6V  --> SP- and P1
SPV --> P2

Nothing on the 5V 7I96 side should connect to the spindle drive

I think I understand, I thought the SPINX1 had those as output, so I need to hook up power there. as well.
Like this:
 

SPV (the variable 0 to +6V voltage) goes to P2

You mean like this: