While this is true (and the one you posted was very, very old), you have to know how to read it properly.
First, a word of caution: you are not derating the conductor to 80% when you consider continuous loads. Instead, the rating of the circuit, after derating factors, must be sized at 125% of the load. It makes a difference when you need to stack other derating factors in the mix or combining continuous and non-continuous loads. Most of the time it works out the same way, but the distinction can be important - just a nit-pick.
Ok, so let's talk about that table you posted. There are two factors you need to worry about: conductor rating and termination rating. If you need to use derating factors like ambient temperature correction, you do it against the conductor rating, but then the maximum circuit rating is the lower of the two factors.
For example, that table will show you that #6 is good to 55A (60 degC), 65A (75 degC), and 75A (90 degC). Most wire in conduit - THHN/THWN - has a 90 degC insulation rating. However, almost all residential equipment (NEMA receptacles, panelboards, etc.) has termination ratings at 75 degC. This means that a circuit relying upon #6 can never exceed 65A (even though the conductor's rating says it's good to 75A).
