First of all, I'll have to determine what cap is. One rating is caped when it has reached the maximum number/percentage. For example reach 100% critical chance, when all of your attacks are critical. As you already know most of the rating have a diminishing scaling when you stack them. What I'm going talk now is soft cap. Soft cap is the point where a stat suddenly becomes less useful and another stat becomes more valuable.
Here is the graph that I found on mmo-mechanics:
As you can see, from a point and further, stats like surge and critical etc have a breakpoint, when if it reached, it stops to be optimal. Here:
SOFT CAP
Surge: 300 rating
Critical: 450 rating
An example to what soft cap, is surge rating. At 200 rating, you have 72% critical multiplier whereas 250 rating rise it to 76%. That means that the 50 rating that is spend to surge could be another stat that would benefit you more.
A general small guide to gear up is:
Accuracy, up until 100% cap
Surge, up until 300
Crit, up until 450
Power
Power on the other hand has no cap. In general you should know that power boost your baseline attack/heal and is a stat that increase your performance directly. I would just healer to focus more to power than surge/critical as they should be depended on RNG critical heals.
Finally, a small tip. You can get critical not only from your critical rating and your main stat but also from some of the base stats of other classes.
WARRIOR/KNIGHT: Strength AND Willpower
BOUNTY HUNTER/TROOPER: Aim AND Cunning
INQUISITOR/CONSULAR: Willpower AND Strength
IMPERIAL AGENT/SMUGGLER: Cunning AND Aim
So, if you are a min-maxer, don't forget to add some more datacrons to your hunting!
