I don't have a prescribed dosing method. I only use it for an anti-snail dip on new plants, so keep that in mind when you consider my recommendation.
I add permanganate to my dipping bowl until the water is a medium to dark pink. It takes only a very small amount to reach that concentration. I usually leave plants in the dip for fifteen to twenty minutes. So far, I haven't introduced any unwanted snails to my tank.
A precaution: permanganate is a strong oxidizer, and also capable of causing serious brown stains on anything you might get it on (the MnO2)
Be careful with it, and don't let it come into contact with things that might burn. Wood, sawdust, sugar, you get the picture.
edit: this link shows a picture of a solution which looks about the same as what I use for a snail dip.
another edit: Your Ebay seller shouldn't be shipping KMnO4 in a plastic baggie. It could break.