Your SPG reads in Bar pressure units for the particular cylinder that you're actually using and breathing at depth, so RMV in Liters is not very useful in that respect.
For example, a RMV of 22 liters/min per ATA with an Aluminium 11 liters/bar (AL80) cylinder yields a SAC of 2 bar/min per ATA: that is RMV divided-by Cylinder Rating equals SAC -or 22 liters/min per ATA divided-by 11 liters/bar equals 2 bar/min per ATA.
Therefore at 20 meters depth (3 ATA), you will consume three times as much of your SAC: or 3 ATA times 2 bar/min per ATA equals 6 bar/min.
Hence you know in ten minutes at 20m depth, you should nominally consume 60 bar in that 10 minute time interval; so if you started with 200 bar reading on your SPG, you would expect it to read 140 bar remaining at the end of ten minutes elapsed time (and 80 bar after another ten minutes; and finally 50 bar remaining after five minutes in which you should be already ascending to your safety stop).