One thing about the extra overflow container is that as long as I can see some fluid when hot, then I do not worry about the level. You can fill both the original expansion tank and the extra overflow container to the point that you are forcing fluid out of the extra tank onto the ground.
You will need to find the fluid level in the original overflow tank that allows some flow to the extra container when hot, so you have a visible fluid level, but not so much that it overflows. In my case, which is linked in Gerald's post, I cannot start with a full original container and not have the added second container overflow. I think I run the first tank about 2/3rds full to have the hot level indication in the second tank. You will find your sweet spot and not be wasting time and antifreeze anymore. The 1 gallon version, if you have the space may be a little easier to manage but not at all necessary.
Later Ed
