As long as you don't change the size of the largest cog on your cassette and the largest chainring on your crankset during the life of the chain, here is a summary:

The chain should be sized by running it around the cassette's largest cog and the crankset's largest chainring, without running the chain through the rear derailleur, and finding the shortest point on the chain where the chain could be connected together, and then add one inch (from one pin to the other is one half-inch, so skip one pin and go to the next to get the inch measurement).

If, in doing this, you are "between links" in finding the nearest point where the chain can be connected, size up rather than down; better to have a chain that is a half inch longer than it needs to be than a chain that is a half inch too short. This chain sizing method is often referred to as the "big-big plus one method," and will give you the shortest chain possible to work safely with your drivetrain. Keep in mind that the most important rule in chain length sizing is that the chain absolutely must be at least long enough to safely shift to the largest chainring/largest cog combination. If not, even if you are determined to never shift to that combination, all it takes is one brain lapse and you will likely do major damage to your bike.

If you use the "big-big plus one" chain sizing method, and find that your chain is slack when in the smallest chainring and smallest cog(s), then your rear derailleur doesn't have enough chain wrap capacity for your drivetrain. As you have found, the result of this is usually not a big deal. Those gears are rarely used anyway, and the bike will usually still work okay unless it is an extreme case of too much chain for the derailleur to handle.