So I am using D4 for spindle speed which is the default for a RAMPS/mega2560 combo I believe. That appears on the second set of “servo” pins on the RAMPS board, second set from the reset button. The D4 pin is used because it is specifically a PWM capable pin I believe. I haven’t tried switching to using the D8 pin. Looking at the schematic for the RAMPS board, it looks possible to use D* to get a 12V signal, BUT it would still be a PWM signal. You would still have to convert it to a DC voltage for the 0-1V input on the spindle power controller. That could probably be done by sending the signal through a resistor to a capacitor to filter it to smooth (enough) DC. It may also be possible to choose a resistor value high enough that you end up with 10V DC out of a nearly 100% duty cycle 12V PWM signal if you get what I am trying to say.
Something like this
With the D8 input on the left and the 0-10V output on the right. Not sure what the values might be. I may get a chance to experiment with this at some point to see.
That said, I REALLY like the idea of my spindle and my arduino being opto isolated so I will be trying the off the shelf converter like the one mentioned in @robertbu post below (above?)