It should be approximately accurate, or at least repeatable in where it changes from full to 2/3 to 1/3 to empty, but clearly not very precise at all. There is a gauge on the tank itself that is more useful but often not the easiest to see, in as much as there's usually a sender module for the control panel's gauge covering the middle of the tank gauge's dial.
The tank gauge itself is based on a simple float arrangement inside the tank. Note that there must be some headspace in the tank, so the 80% level on the tank gauge is full. Whether the inside gauge is calibrated in terms of usable capacity or tank capacity (or indeed is calibrated in any practical sense of the word) is anyone's guess without making some tests.
I find mine is handy as a general guide for when I should start thinking about getting more propane. A full tank lasts a good long time if you don't use the furnace (or generator, if you have a propane fueled generator), a number of weeks in my case.
