For the short term, I would suggest weighing the tank and reg. The tank will have a tere weight stamped somewhere near the top where all the markings are located. This will be the empty tank weight and by weighting without the reg, etc. you can tell if you have a full tank of 20 or a near empty weighing only a couple pounds more than the tare weight.
To avoid having to remove the reg each time to check the level, I would also weight the reg, add the two readings together and that would let me know how much is left by simply weighting both together.
Point to keep in mind is that after the liquid portion is gone and that is what you weigh, there will still be a fair amount of gas left. Since you seem to be using a low quantity, I might expect the gas portion to last in the range of two weeks. But that is also very much a guess and you should not trust my guess too far!!!