As "Pinny" mentioned, your air consumption should be in cubic feet (or liters) per minute and NOT PSI per minute because PSI is inaccurate. For example you can breathe 200 PSI in a pony bottle in a couple minutes while it would take you 5xs longer to breathe 200 PSI in an 80 cf cylinder.
As a technical diver, I have to know my SAC (Surface Air Consumption) rate to calculate how much gas I will need for a tec dive.
Here is the formula I use:
[[(psi used/working pressure of tank) X total cylinder capacity]/[(depth in feet + 33)/33]]/amount of minutes
In "English..." take your used PSI and divide it by the working tank pressure. Multiply the number you get by the total cylinder capacity and you will get another number, lets call this "X." Next take your depth (in feet) and add 33. Take the number you get and divide it by 33, we will call this number "Y." Next, divide "X" by "Y" and you will get another number called "Z." Divide Z by the amount of minutes and you get your SAC rate!
So say you want to see your SAC rate at depth. At 60 feet (or whatever), level off and record your starting pressure, go along your dive for lets say 5 minutes then record your ending pressure. Subtract these two to get how much PSI you used, "psi used." Next check the working pressure of your tank on the tank stamp, usually it is 3000 "working pressure of tank". "Total cylinder capacity" will be how much cubic feet it holds (ex standard tanks are Aluminum 80s so 80 cubic feet).