So I've found this method to be great in keeping an eye on my propane usage.
Take a look at the tank and you should see something that says TW - That is the tank weight of an empty tank. Weigh the tank and subtract the tank weight. That will give you your propane weight. Dividie that by 4.23 to get your gallons of propane. Then multiply that by 91500 that will give you your BTU's remaining. You will need to know your burner rating in BTU then you can divide by that to give you the hours remaining. Example
TW - 17.2
Current weight - 22
Propane weight - 4.8 LBS
Gallons of propane (weight /4.23) 1.134752
BTU remaining(gallons *91500) 103829.79
Hours remaining (BTU remaining / burner rating, in my case 15000) - 6.92 hours
I hope this helps. I strung this together from posts here and some more research on Wiki-pedia
I also write on my tank with the approx # of hours remaining as I weigh it after I'm done smoking. I also keep a spare but at least I'm ready to go roughly when the tank runs out .