Here is how I've been doing it. 3.4v 1.7A = 5.78W --convert that to the 12v we will be using -5.78W / 12V = .48A -then de-rate it a bit to 90% (don't want it to get too hot.- .48A .9 = .43A Each.
I these are pretty close to my steppers. So I set the Z pot to .4v and the X and Y to .7v Any more than that and you really need a fan always on the heatsinks. If the steppers are getting too hot turn this down a bit ( I aim for warm) if they aren't warm you can get more power out of your steppers by turning this up a bit. Too high and you will over heat or miss steps all together.
This is only for the DRV8825, the a49's are different.