Help with correct vRef (no replies)

Back story is I have had my printer for few years and I was told not to worry about setting it with a meter just tune it to where it doesn't skip steps and leave it. I have had nothing but issues with my extruder where the hobbed gear slips I think maybe due to the extruder being too hot causing the slip. It is fine if its an 30min print but any longer its it will slip for sure.

I have ramps 1.4 with china pololu 4988. The resistor values on the board say R100 a pair of them so I assume that these are the sensing resistors. (I can not read the board to see if it is r7 and r8 its smeared)

The motors I am using are 1.8a/phase (if I am reading it right.)
X, Y, Z
[artofcircuits.com]

And for E It just says 1.6a I don;t know if that is per phase but I assume.
[www.phidgets.com]


So here are my questions.

1. Would this be the correct way to figure out my vref for 90% of max?
1.6 * 8 * 0.1 = 1.28 * 0.90 = 1.152v


2. Is 90% too much? What do you recommend?


3. Do I have to double the value of the Z to provide it twice the power since it is running 2 nema's?

Thanks in advance for your help