How Much is Left in That Paint Can?

It doesn't take a lot of calculation to check the level with a dipstick and write on the can how much is left. This can holds the UV-resistant varnish I use on my back door - it's good for a number of years between applications and the job only takes about 4 ounces. Now, all I have to do is look at the can on the shelf to see that I have enough for the next time:
