Bottom gas is 10/50. Deco gases are 21/30, 50/10, and O2. Target depth is 300', with contingency allowance for 330. I don't mean contingency as in, we might change the dive plan mid-dive or get in with a plan for 300, but possibly go to 330. I mean contingency as in filling tanks today for this weekend and, for Monday's dive, we might end up deciding to plan it for 330, using the gas I already filled my cylinders with.
I did not choose any of the gases. They were communicated to me by my instructor. That said, the idea of switching to 10/50 at 30', and staying on it to 20', then switching to O2 makes me very uncomfortable. Or whatever your process is. Being on 10/50 at 20' is not something I really want to do.
The class is TDI Advanced Trimix. Yes, open circuit. I had not planned to pursue hypoxic OC training, as I am really only interested in this kind of deep diving on CCR. But, the opportunity came along to do it at the right time, for the right cost, so I decided to do it. I'm working towards becoming a tech instructor and this training and experience seems like a good thing to have under my belt, even though I don't expect to ever get to the point of teaching hypoxic trimix on OC myself.
I have practiced switching directly to one deco gas from another and it didn't seem any harder than switching from back gas to a deco gas. Maybe easier. There's a lot less gas switches that way too, which seems like a good idea. And no frequent clipping and unclipping of the primary back gas 2nd stage.
Once I switch from back gas to the first deco gas, the primary back gas 2nd gets clipped off and stays clipped off. With it clipped off, the alternate on its bungee, and a deco reg around my neck and into my mouth, it was pretty straightforward to deploy the next deco 2nd stage, confirm the gas, etc., etc., and then take the 1st deco reg out (and from around my neck) and put the 2nd one in - all without tangling the hoses, of course. Maybe I just don't have enough experience with that yet, to see the true shortcomings.
Anyway... that was to answer some questions raised, but is all OT.
From the responses that addressed my questions, it sounds like I should probably be fine with my Kydex BP and the same tail weight I was using this past weekend (i.e. 5#). We'll be doing some shallow skills dives before we actually really go deep, so I will have plenty of opportunity to make adjustments and practice all this. I just came here to consult the Scubanati to get some validation of my plan for my starting point. Thank you.