With SNHD how is fluid removal calculated and set? Is there ever cramping? If not, how is it prevented?

Fluid removal is calculated the same as for normal 3x4 HDx except that it's now over 8 hours rather than 4 hours.
That means I routinely, every night, remove 250 to 350 mL per hour. You can't get cramps at that rate.

I have been as high as 440 but the nephrologist was not very happy.

Go for it.

How do ml's compare with cc's? What if you calculated wrong and went below your dry weight..what would happen then?

Ml's and cc's are the same thing.

I'm now in center and am considering doing slow nocturnal. I don't have fluid removal (the machine is set in minimum) but on occassion still get leg cramps quite bad. Will that be better on slow nocturnal?