The propane in the tank is liquid. The valve at the top pulls off the vapor and the liquid then boils (cools) to replace the vapor being extracted. While the tank is in use, this process continues. As a result the temperature of the liquid propane drops. The "gauges" that stick onto the side are actually thermometers that will show you this temperature change. The sides of the tank that are not in contact with the cold liquid propane will stay close to the surrounding air temp, while the part in contact with the liquid will cool down. The thermometer strip will show you where that temperature change occurs.
A simple way would be to do it like the "pro's". Simply weigh the tank. A bit harder to do with the tank hooked up, but really simple. Weigh your tank when it is empty using a bathroom scale. Typically they are 20 lb tanks, so they should weigh 20 lb more when full. The capacity is stamped on the ring around the valve. When you are getting close to the weight of the empty tank, it is time for a refill.
A pressure gauge will not do you much good. The pressure in the tank will stay pretty much constant (vapor pressure of the propane at ambient temp) until all the liquid is consumed, then the pressure will drop rapidly.
I just pick up the tank and give it a shake to "guestimate" the amount remaining. I also try to keep a second tank on hand that is filled. Note the "try". I just recently had to run to the gas station and do the exchange because I forgot to turn off the grill and ran it empty. A gauge will not help you prepare for those situations.
Sorry for being long winded and not necessarily giving you an answer.