I dove till 2013 without computer and if needed I still can do.
First option:
-Make a plan with Vplanner/Multideco. Adjust it where needed (Pragmatic deco). The +5m depth or +1-3 minute bottomtime was easy (depending on depth). For example, if you go to 100m and one minute more bottomtime means about 8 minutes more deco. So do 4 minutes at 6 on ean80 or 100% and the other 4 minutes between 21m and 9m. So that will mean 1 minute 18m, 1 minute 15m, 1 minute 12 m and 1 minute 9 m. OR: 2 minutes 12m and 2 minutes 9m. Deco is no absolute science.
At 120m is 1 minute 20 minutes more deco, so 10 minutes 6 m and the other 10 minutes between 21m and 9m.
The examples above are just examples, and some of the limits are: deco is not lineair, but if you talk about 1-2 minutes more bottomtime at 100m, the results are more or less linear. If you use this, you only need plan in your wetnotes and the note: +3m =+10 minutes deco, +1 minutes=+8 minute deco (examples too).
Pragmatic deco can be used too as you know the bottomtime and the total divetime. If you forget all other things you still can make a decoplan on the fly to give you a good option to come healthy out of the water.
Second option:
-Ratio deco. Here you use more or less the same as in the example above. 40 minutes deco means 50-80% of time on the highest decogas, rest on lower decogas. But best is to prove it with software. Pragmatic deco is more or less same as ratio deco. Ratio deco is used with standardgases, but pragmatic deco can be used with every gas.
Third option:
-Average depth calculation. This is the option that is less trustable. I always said use this as last option when you forgot all other things. Average depth is tricky in caves too. On normal profiles you willl see your average depth of a trimixdive will be between 22 and 26m.
All options above work easiest on 'easy' profiles like a wreck. Average depth in a cave can be totally useless (you can reset it on a bottomtimer, but even at 20m depth you will build up nitrogen and if the way back is long time at 15m maybe your deco is not gone at the exit, but the average depth shows it is fine).
To use the first option in a cave (pragmatic deco) you need to have the map to know depths and lengths, know your swimmingspeed in and out, etc. The first option is always used by me, even if I know have 2 trimixcomputers. I always plan a new dive with software. Why? I need to know the amount of gas/bailout needed. The only difference is that during a dive I use the computer and not follow the wetnotes plan. In a course divers have to follow the wetnotes and are allowed to check it with their computers. But in my eyes every diver need to be able to plan and execute a dive without computer.
When experience grows you will see globally if a diveplan is possible or not even without software.
First option:
-Make a plan with Vplanner/Multideco. Adjust it where needed (Pragmatic deco). The +5m depth or +1-3 minute bottomtime was easy (depending on depth). For example, if you go to 100m and one minute more bottomtime means about 8 minutes more deco. So do 4 minutes at 6 on ean80 or 100% and the other 4 minutes between 21m and 9m. So that will mean 1 minute 18m, 1 minute 15m, 1 minute 12 m and 1 minute 9 m. OR: 2 minutes 12m and 2 minutes 9m. Deco is no absolute science.
At 120m is 1 minute 20 minutes more deco, so 10 minutes 6 m and the other 10 minutes between 21m and 9m.
The examples above are just examples, and some of the limits are: deco is not lineair, but if you talk about 1-2 minutes more bottomtime at 100m, the results are more or less linear. If you use this, you only need plan in your wetnotes and the note: +3m =+10 minutes deco, +1 minutes=+8 minute deco (examples too).
Pragmatic deco can be used too as you know the bottomtime and the total divetime. If you forget all other things you still can make a decoplan on the fly to give you a good option to come healthy out of the water.
Second option:
-Ratio deco. Here you use more or less the same as in the example above. 40 minutes deco means 50-80% of time on the highest decogas, rest on lower decogas. But best is to prove it with software. Pragmatic deco is more or less same as ratio deco. Ratio deco is used with standardgases, but pragmatic deco can be used with every gas.
Third option:
-Average depth calculation. This is the option that is less trustable. I always said use this as last option when you forgot all other things. Average depth is tricky in caves too. On normal profiles you willl see your average depth of a trimixdive will be between 22 and 26m.
All options above work easiest on 'easy' profiles like a wreck. Average depth in a cave can be totally useless (you can reset it on a bottomtimer, but even at 20m depth you will build up nitrogen and if the way back is long time at 15m maybe your deco is not gone at the exit, but the average depth shows it is fine).
To use the first option in a cave (pragmatic deco) you need to have the map to know depths and lengths, know your swimmingspeed in and out, etc. The first option is always used by me, even if I know have 2 trimixcomputers. I always plan a new dive with software. Why? I need to know the amount of gas/bailout needed. The only difference is that during a dive I use the computer and not follow the wetnotes plan. In a course divers have to follow the wetnotes and are allowed to check it with their computers. But in my eyes every diver need to be able to plan and execute a dive without computer.
When experience grows you will see globally if a diveplan is possible or not even without software.