SS increases your DoT/HoT ticks by the same percentage that it decreases your GCD. 1% off cast time (~64 SS) also gives +1% to each DoT/HoT tick.
Crit hit rate starts at roughly 5% with each critical hit giving a +45% increase in damage. For each ~43 Crit you add each of those numbers increases 1%, meaning it's not a straight line.
You want a simple y = Ax equation, but that's not how stats work in this game. Weights are given at a rough value in current top end gear, and for healer they would vary between healing and dps. The best you will get is how the stats weigh for someone else's gear and playstyle.
