Burton on Trent water profile

I'm trying to use Beersmith to go from Washington, DC water to Burton on Trent water for a quantity of 15 gallons of starting water.  I've got gypsum, epsom salt, table salt, and baking soda.  I can get pretty close with 60g of Gypsum, 14g epsom salt, and 14g baking soda.  That seems like way too much gypsum, though.  I got a little ID Carlson jar of gypsum from a homebrew store, and the entire jar is only 54g (2oz).  

I know BoT water is hard, but that seems excessive, even for 15 gallons.  Can the calculation in Beersmith be right?

Eudmin,

I use  Fairfax County water (6 miles from DC) and find that it does not matter what I do, but I cannot get the profile down to something useable.

I usually use 3tsp for 8 Galls of Mash water, and even if I add 5.2 buffer in large amounts, I still am getting a pH in the high 5s, not the low end.

I would be interested in other local brewers telling their secrets of Mash pH balancing and what they use, as I am after English Ale styles predominantly .

The actual composition of BoT is useful, but getting the pH right for the Mash is the paramount concern - After the mash, it is less of a problem - and the initial hardness.  As I understand it, the pH of the water and the overall hardness affect what malt profiles you can get away with.  The Palmer book - how to brew - is a lifesaver at explaining this, to a level that I can understand but not regurgitate.

You might find it easier to boil the water ahead of time and siphon off to get rid of the excess chalk, then try to add the BoT salts to that.  Filtering it does not seem to remove much other than the chlorine and I am not ready to invest in an ion exchange filter yet.

Roger