First there is no question 500 mg is safe, I and others on this forum have been over 1000 mg/day with no ill effects.
Now lets address easier;
Vitamen C, at 500 mg of Lugols
your going to need 5-10 grams per day spread out over the day, just like the Iodine
perferably 1-2 hrs before or after Iodine.
Selenium 400-800 mcg per day spread out over the day.
R-Lipoic Acid 150-300 mgs per day take with C and Selenium is OK
Niacin B-3 200-500 mgs I use this 2-4 times a week to open-up the small veins and allow the Iodine
to flush through, its a real rush.
IP-6 1.5 -3.0 grams per day, taken on empty stomach, morning, afternoon, evenings at least 3 hrs after eating or 30 mins before.
All the above will assit in speeding the metals, bromides, pathogens, dislodged by the Iodine
out of the body, there by making things less stressful on the elimination organs.