How did you arrive at such a small range of crit rating values?

Math, complied data, the healing forums and the plausible combination of pieces using 63 slots. You can only get 340, 352 and 354 as it is without shooting to 381-396 which digs into the curve very hard. Once you go over 352 you hit the curve at a point where power becomes more optimal. This is for a number of reasons, but the greatest is because the expected modifier from your crit and surge starts to turn in less when applied to the coefficient of the healing powers involved.

You can test it out on a parser, but as long as we don't hit the awful stats of 2.0 this is literally the best combination of secondary stats a sorc/ops can have. Some argue that stacking overkill augs is a good idea, but I say bunk. Willpower is the greatest stat to have currently as a healer as it gives 0.14 healing power and its critical rating does not compete with your critical chance stat. DPS wise overkill is a godsend.