I didn't say 'always start in the 90 degree column' ... I said "assuming the wire is a 90 degree type ...."
Where does the code say so? What the code says is that the table lists the ampacities of the different wire types. That is, for example, that #12 THHN can handle 25 amps.
That's your starting point.
THEN the code lists various derating factors .... that is, circumstances where the wire is no longer allowed to carry 25 amps ..... things like ambient temperature being higher than usual, conduit fill, voltage drop, etc. These things are used to reduce from that (in our example) 25 amp figure. Finally, the table has a note that says "don't use #12 with a breaker larger than 20 amps unless it's a special case."
Finally, you look at how many of those 25 amps are left after all your calculations. If that figure is higher than the figure in the 75 degree column, you need a larger wire ... so the wire won't get hotter (from carrying current) than the termination at the device can handle.
Look at it this way: you can't often start at the 75 degree column, then 'derate' your way up to the 90 degree column.
The key is to istall the wire in a manner that will ensure that it never gets hot enough to damage the insulation. Code reference? You'd have to go back to 110, and the 'follow instructions' requirements. If the wire is listed for only 75 degrees (for example, THWN actually used in a wet location), you would have to start your calculations from the 75 degree values.
