how much potassium metabisulphite to use

I have done a couple of ciders and have always used the directions on the bottle of meta for the amount to use in a 5 gallon batch, 1/8 tsp IIRC.  However, I was perusing the Wittenham Hill Cider portal and he recommends using the pH of the cider to calculate the amount of meta to use http://www.cider.org.uk/frameset.htm.  My current cider’s pH is 3.8, so for a 5 gal batch it recommends adding 15 campden tablets.  That seems like a huge amout of meta and am hesitant to use that much.  How much does everyone else use