Machine Tuning/Voltage Question

I'm trying to spend as much time as I can learning about my machines. I don't want to (eventually) be the kind of chik or tattooist in general that doesn't know about my machines. I'm using the book "Tattoo Machines And Their Secrets" I tuned both of my machines - My Apollo, which I use as a liner. And my Strongman that I use as a shader. On both machines, I adjusted my front binding post so that more of the screws surface touches the front spring. I adjusted the contact screw on each machine and adjusted the springs so that, like the book says, when I do the "flex test" the armeture bar only moves about 1/16th of an inch. I also, for the first time, was able to get the right response from both machines using the "Ball Test". However, when the "balls" look correct (closer together for my liner and a little further for my shader - just like the diagram) Both machines have an extremely quiet hum and each run at under 4 volts. Is this way too low? Did I do everything wrong?

I should add, this is without the machines being set up with needle, tubes, rubber bands...