[cash]

My water analysis is as follows: Ca 6, Mg 2, Na 18, SO4 18, Cl11, HCO3 35, T. Hardness 23 , Alkalinity 29 ,RA 23 SO4/Cl Ratio 1.6
  I am going to brew(all grain) an Imp. IPA using 16.5 lbs maris otter and 1lb crystal10(SRM~9.4)In using Bru'n Water I tried to match the Pale Ale profile as closely as possible.I am very new to water amendments so I would really appreciate any input/suggestions about the following water amendment proposal.
Gypsum 1.25( gm/gal)
Epsom .50
CaCl     .25
Chalk    .50
Phosphoric acid .5ml/gal

[thcipriani]

I know that you're trying to match the "Mosher's Ideal Pale Ale" profile - and if that's the goal, your additions are probably fine; however, I am uncomfortable putting that much chalk into a pale beer like the one you're brewing. In my experience Crisp MO has given me some fairly high pH readings - even with the amount of acid you're adding I feel like you're running the risk of shooting a little North of the recommended pH range.

If I were doing this beer, I'd probably drop the Epsom, Chalk and Phosphoric and just try it with a small amount of CaSO4 & CaCl2 (enough to get to >50ppm, allowing for some margin of error). I brewed a beer last weekend that used 1lb of Simpson Dark Crystal (75L) and 1/2lb of Special B (120+L) using Rahr 2-row base (typically fairly low dough-in pH) and I still had to add acid malt to get into the right pH range. My point is that I'm more often at a higher pH than I'd like to be and adding chalk before I dough in, to me, seems like playing with fire.

I am likely in the minority on this, FWIW.

[hopfenundmalz]

I agree with Tyler.  You are using chalk, and have to add acid to pull the pH down.  Why not leave out the chalk and use less acid?  Keep it simple on the water additions.