I am a newly certified technical diver with TDI in Israel and had some questions about dive planning that are not really answered in the manual. We are doing open circuit, open water dives to 45 m with 2 11 Lt tanks (doubles) usually filled with EAN25 (no helium) and a standard deco bottle with EAN50. We would like to stay at our bottom time for 25 mins. I am trying to figure out how much gas I need to bring with me when considering contingencies (we don't have to get back to a start line).
The TDI manual gives examples for dive planning regarding calculating how much gas reserve you should have and what your turn pressure should be (I can email you exert of manual if interested). In their example they do a dive to 30 m on air for 45 m (Vplaner then gives them deco stops with a runtime at 93 mins). The book examples (pasted below) are then based on that hypothetical dive. The problem is that all these examples in the book assume you are not using an additional deco bottle like we are. I provided text below of what is in the manual (rules) and example calculations for their theoretical dive . But what I would like to know is how would I make similar calculations following their rules (see below) when I have a deco bottle? What "rules" should I be following to make sure I bring enough gas and follow a moderately conservative profile? For example, should my gas reserve calculation not be based on getting to the surface but to my gas switch? Should I factor the gas switch into calculating my turn pressure?
I didn't find the right solution from the Internet.