I generally use Lugol's in my last dipping tray before I put frags in the tank. I use 3 trays/containers when adding new frags and they all contain about 1000mL or 1500mL of water in each. 1st container gets 5mL to 10mL of Melafix or bayer (forget dosage on bayer) and dip for no more than 5 minutes. After that the frags go into the second bath of clean saltwater to rinse off any Melafix/bayer, usually another 3 minutes or so, then in the final container which contains fresh saltwater from the tank with a little Lugol's (about 7 to 10 drops) and let the frags sit in there for a couple minutes, then into the display.
If you're just dipping with Lugol's, then it's 40 drops per gallon for about 10 to 15 minutes. It's roughly 1 drop per 94mL of water (if that helps?).