Current formula for TMC2660

So I have searched for a formula for setting the current of these drivers but can't find any. Do people use the TMC2100 formula?

You can set the current directly using M906:
https://duet3d.com/wiki/Gcode#M906:_Set_motor_currents
Example
M906 X300 Y500 Z200 E350:350
Sets the currents to send to the stepper motors for each axis. The values are in milliamps. 
Thanks, I saw that and am excited over turning with screwdriver. What I have yet to find is the formula for figuring out what mA to start with. Like other drivers have you set a Vref based on max holding current * 80% * 1.x
So I guess since this is straight current then a good start would be 80% of max holding current???

Yes.