Allow me to be a contrarian and suggest a way that may come close to a solution. It is to accurately determine how much fuel is in the tank when you first see fuel under the filler hole. The determination may require you to drain a tank.
I'm on my second Skymaster (the first a 1969 337D with 4 filler caps, now a 337G with 2 filler caps), I know a bit about our fuel systems (see my "Fuel Supply Management" page at
www.SkymasterUS.com), and because I fly mostly over water to a remote island where there is no 100LL, fuel is especially important.
In my 337G, I determined that at 30 gallons is when you first see fuel under the filler hole. You have to be on level ground, with the tank on the other wing at about the same fill, and allow a minute or two for the fuel to settle. Obviously, this method doesn't work for fills below that "just-see" amount, but it's great for greater fills: just add however many additional gallons (if any) you need.
Incidentally, I am
critically dependent on this trick and use it on every trip to the island. You might deem me paranoid but I am concerned about falling out of the sky on the
return trip, because someone stole fuel from the airplane over the days or weeks it sat on this island -- where automotive fuel is expensive, security is minimal and the people are poor. So, when I leave Miami, I first fill to the "just-see" mark and then I add the amount I need to get to the island plus a little more (note: can't add indiscriminately because we're typically at full load outbound.) Then, when I do the pre-flight check for the return trip, I climb on the wings and make sure I see fuel on both tanks.
Ernie