Inaccurate gas gauge

I use a 6.5-gallon plastic, portable gas can in my G3 1544 and until recently it has been reasonably accurate. A couple weeks ago, however, I noticed that it wasn't even close to showing what was really in the tank. When the can is full it now shows that it's about 1/3 full. I checked the float and side rails and the float runs up and down easily so I don't know what's going on or how to fix it. Anybody got any ideas?

Along those same lines, I've never been able to figure out how much gas actually remains in the gas can so it's hard to top it off. Just by opening the cap and looking inside I've never been able to guess how much gas to add, so I came up with my own solution. I emptied the gas can and began adding gas a gallon at a time. As each gallon was added I set the gas can up on my work bench and put a strong light behind it. Once the gas had quit sloshing around, the light allowed me to see where the top of the gas was. At this point I marked it with a magic marker and scratched a line across it in case the magic marker wore off. Then I added the second gallon and repeated the process. I kept doing this for each gallon until the gas can was full. I then took a Dremel tool and engraved numbers at each line to show how many gallons of gas were at each line. Now all I have to do to get a real good idea of how much gas is remaining is hold the can up to a strong light. Works great. If anybody's interested I'll send photos.

I mounted a tank under the front deck. I use a wooden ruler to measure how much fuel is in the tank.

I did that too but because the gas can isn't uniform in shape from top to bottom, halfway up the stick didn't mean I had a half tank of gas. The way I marked the gas can using actual measured amounts of gas takes into account the non-uniform shape of the can.

My tank is a nice rectangle. The ruler works well for me.