I recently had a friend ask me how he could tell if his 20 lb. propane tank was near empty. I own 2 tanks so I don't really worry about it. So in searching for a good answer for him, This is what I came across from Wikipedia.

The only accurate way to measure the propane left in a propane tank is to weigh it. Engraved on the side of the tank should be the letters TW then a number. This number is the empty tank weight. For example, the typical 5 gallon propane tank might say TW 17.2. The weight of the empty tank in the example is 17.2 lbs. Next, weigh the tank on a bathroom scale to find the total current weight. Let's say that the current weight of the tank is 24.2 lbs. To find the weight of propane simply subtract the tank weight from the total weight.
24.2 lbs - 17.2 lbs = 7 lbs
Each gallon of liquid propane weighs 4.23 lbs. Divide 7 by 4.23 to get the number of gallons currently in the tank.
7 ÷ 4.23 ≈ 1.66 gallons of propane

I use a GOSM big block with a BTU rating of 18.000. Based on my spreadsheet I'll get about 5 hrs. burn time per gal.
I tried this and it worked for me. I hope you can benefit from this.