How much extra anesthesia would someone need if their hair is not red or brown but halfway in between (auburn), compared to brown hair?

Not known: In the studies, which were small, involving redheads, there wasn't particular stratification based on degree of redness. There have been later studies which suggest the increased anesthesia need may not be as significant(or even at all) as thought.

Not a factor: Your general anesthesia needs are usually determined by your body wt., how much of that wt is fat, and if you are a smoker or drinker. Smoking, for example, can affect the Cytochrome P450 superfamily of proteins, and can lead to more rapid metabolism of both local and general anesthetics. A fairly recent study in 2012 published in the Anaesth Intensive Care, 21012 July, 41(4): 683-9 covers this.

Anestheisa and Hair: I do not believe there is an association with hair color and the amount of anesthesia one would require

There is: An association with anesthesia requirements with ethnicity but not hair color per se