The method is to swim for 10 minutes at a fixed depth, record tank pressure before / after and that will give you your RMV rate. It will never be quite precise as you tend to swim a bit more when you measure your RMVrate. Alternative you could stay at a fixed depth and do nothing for 10 minutes to get your "deco" RMV rate.
The point is to get an approximation, you will round it anyway quite a bit to get something easy to calculate underwater.
Eagle freedom units don't help here but you can most likely flip a coin - your RMV will be either close to 15 lpm (litres per minute) or 20 lpm (* for the bottom portion of the dive and perhaps either 10 or 12 lpm for deco.
Let's assume it's 20 lpm. RMV is not very useful, so let's calculate what would be your SAC rate - how much pressure in bars would you consume every 5 minutes out of your cylinder on the surface.
11 litres cylinder (ali80): 20/11 * 5 = 9 bars every five minutes on the surface, so le'ts round it to 10 bar
12 litres cylinder: 20/12 * 5 = 8 bars every five minutes on the surface, still round to 10 bar
15 litres cylinder: 20/15 * 5 = 6.5 bars every five minutes on the surface, so about 6 bar?
2x12 litres cylinder (doubles/twinset): 24/20 * 5 = 4.15 bars every five minutes on the surface so let's round to 4 bar
If I assume that I'm diving at around 30 meters (4 atmospheres pressure) on a 2x12; doubles, I should be consuming about 4 atm * 4 bar (surface) = about 16 bar every five minutes. Underwater I would round it to either 15 bar every 5 minutes (looking at pretty fish) or 20 bar (swimming a bit). So if you start your dive with 220 bar pressure, 10 minutes (= 2 five minute intervals) into the dive you should see somewhere between 190 bar (220 - 2 intervals * 15) and 180 bar on your SPG. If you see more - cool, your SAC rate is probably lower and maybe you could switch to 15lpm. If it's a bit higher - maybe you are a bit nervous on the dive, maybe there's a bit of current.
There isn't much of point in trying to be too precise, because each dive is a bit different. You need to find a set of numbers that are easy to use underwater - mental math is not fun when narked or taskloaded
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(* if your SAC/RMV (can' rate is more than 20 lpm, go work on your cardio/buoyancy/fin kicks/underwater comfort)