I hope this might be of some help:

Alpha-lipoic acid dosage, intake, recommended daily allowance (RDA)

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Currently there are no established recommended doses for supplementation. For use as a general antioxidant, a lower dosage of 20 to 50 mg daily is commonly recommended. Recommendations for the use of racemic alpha-lipoic acid as an antioxidant range from 50 mg/day to 400 mg/day. The typical dosage of oral lipoic acid for treating complications of diabetes is 300 to 600 mg daily. Alpha-lipoic acid can be purchased in dosages ranging 30 mg to 100 mg tablets.

Sources of alpha-lipoic acid

Alpha-lipoic acid is mainly derived from dietary sources. Most alpha-lipoic acid in food is derived from lipoamide-containing enzymes and is bound to the amino acid, lysine (lipoyllysine). Good food sources of alpha-lipoic acid include spinach, broccoli, beef, yeast (particularly Brewer's yeast), and certain organ meats (such as the kidney and heart). Animal tissues that are rich in lipoyllysine include kidney, heart, and liver, while plant sources that are rich in lipoyllysine include spinach, broccoli, and tomatoes, potatoes, yams, carrots, beets. Supplemental doses of alpha-lipoic acid are hundreds of times higher than the amounts that can be obtained from food, and should be considered pharmacologic rather than physiologic doses.

I can understand you wishing to include this as a general antioxidant, but not enough to where you will cause problems with the mercury. I know a few antioxidant supplements that typical have about 10 mg - 25 mg. You may have to try a small amount and test your own responses before increasing.