Is it ~300 crit rating with or without the smuggler buff?

It's worth pointing out while it's good to practice this stuff on the dummy or use in PvP or in smaller groups, if you raid with more than one Sentinel it's nearly impossible to keep track of your burn stacks on the target by looking at the debuff window. I raid in a 16 man with three other Sents so looking at debuffs is worthless. Yeah, I can usually keep track in my head of how many stacks I've applied, but when it gets fast and furious in a boss fight it's easy to forget. There really needs to be an option to filter debuff so that only your own show up on the target frame.

Honestly, I'm not sure how much it matters. Your next 6 DoT ticks are going to crit, so as long as you get all of them before your Zen buff expires (and if you're running the normal rotation I don't see how that won't happen) it should be the same. If you pop Zen when you have all three OS dots running, then theoretically you are going to eat through those Zen crits faster, letting you start stacking Centering again sooner, but I'm not sure how much of a DPS increase that is going to be. The only rule I use is not popping Zen if I only have the Cauterize DoT on the target, since Caut ticks for less than a tick of OS, so you'd prefer to have OS dots crit and not waste stacks on Caut.