The Schaffer's RV Trip Log: Measuring Propane Use
Follow the link above, that's how I measure occasionally to see where I am with respect to the propane amount remaining and what we are doing. Found that running our furnace on these cold fall nights really used the propane, but cooking/grilling was very little. Using a simple scale I can get a good idea. When the popup is closed, I can just loosen the one nut enough to let the tank lift and get a close weight, without having to remove it all the time.
Talking with others at the campground about the screw on meters, some liked them, some didn't. Some noticed a pressure drop on their stove due to the additional restriction it adds, others said they didn't notice. Also, it seemed common not to move much during use, and then Boom,... dropped to red and you were out of propane. Which makes sense, as the pressure would be rather steady till the very end as mentioned by another post above.
The strips that adhere to the sides of the tank can show you the level, but typically need HOT water to activate the strip. Even then, it can be difficult to read sometimes if the water temp isn't hot enough.