Controlling mains 0-9v sherline lathe spindle mesa 7i96s

Hello,

    I am trying to control my sherline lathe spindle. It uses a potentiometer that is wired to mains. I have been looking at older threads for people who have done this but I am still a little unclear on the best approach. 

   It seems mesa 7i96s has a 5-20v isolated analog output - but I would need to convert thisto 0-9v to driver the sherline - what is the easiest way to go about this? I have read that people opted to use a firmware that gives the mesa an extra pwm on stepgen4 and do a pwm to analog conversion board such as spinx1A - but even with the spinx, how do you get its range of 5v - 21 to convert to linear 0-9v? Could you use something like a dc-dc buck converter (xl4015 or LM2596S)? Want to make sure I am doing things as safely as possible.
 

The 7I96S analog output is a potentiometer replacement.
if you supply 9VDC to SPINDLE+ and SPINDLE- the output
will vary between 0 and +9VDC for PWM values of 0 to 100%.

It is expected that the potentiometer connections on the drive
supply this 9VDC.

But the 7i96s manual says that spindle + has to be between 5-20v and spindle- has to be between 5-20V - so how do you get a range of 0-9v from that? Unless the manual means that the difference has to be between 5 - 20v?

Or can you connect spindle+ to 9v and spindle- to negative 9 voltsand use 50% of the PWM range?