Back in the misty past, I stumbled across the part number for the original predator motors, which were rated for 1.68 amps. Given Anycubic's propensity to change things at will, take this with a grain of salt. On my Trigorilla-Pro, the sense resistors are 0.1 ohms, so the Vref for the stock A4988 drivers is Vref=0.8 x Rated current=1.34 volts.
I have my steppers set at 50% of rated current. This is easier on the motors and drivers, they run quieter and cooler, and there is no need to go 100%.
So 50% of 1.34 volts is 0.672 volts.
I have my Predator modified with 0.9 degree steppers, and this has also worked well for me.
Also be aware that the common "clip lead on a screw driver" does not work on the Trigorilla Pro. For some reason Chitu (the board manufacturer) decided to ground the case of the adjustment potentiometer, so you cannot adjust Vref that way. There are three leads on the adjustment pots, only one of which will show an appreciable voltage. That lead is Vref, and you need to probe it while adjusting the pot.
For your second question, I will assume you mean "how did you ground the Rosc pin". You jumper, or short, the Rosc resistor, which is R49 for each of the stepper drivers. A blob of solder bridging the existing resistor will work, or you can get fancy, remove the resistor, and replace it with a surface mount zero ohm resistor. I used the solder blob method, but keep in mind that you are working at very close quarters on very small components.
This is my experience only. I take absolutely no responsibility if you follow it and trash your printer, or burn your place to the ground!