I have been fortunate enough to be hooked up with a LDS that not only teaches, but also promotes TEC diving. These are people that regularly dive 200 to 250 feet. If you can get together with some people like this, that are not afraid to talk and teach you how to do these types of dives, and utilize different than the norm equipment set up. I believe you will have an eye opening experience. It has been for me. I was certified SSI, but the LDS did not have a clue, and yes they all had more dives than I. Imagine an individual showing the instructor the proper tecnique for clearing a mask with a purge. I always use a pony bottle when diving. On dives below 100 feet the pony bottle contains 40% oxygen. This for my deco stops to help off gas. Re-read your gas charts. Claculate your air consumtion rate and start putting it all together. With a 6 minute deco stop at 10 feet you can add 10 minutes to your bottom time at 140 feet. This may not be important to you but it was to me this past weekend. I wanted the extra 10 minutes. I don't know if this helps you to understand. Pony bottles can be utilized for many different purposes, not just as redundancy. Educate don't stipulate!