That's a lot of chocolate malt! I like pale chocolate, but with that much chocolate malt PLUS roasted barley, you may have a very very roasty stout and not one that will take sweetness and flavors well.
How about some more like Jamil's sweet stout from BCS?
(This is a 6 gallon batch, so you can scale if doing a smaller batch)
10 pounds British pale malt (I used maris otter)
1 pound black barley (black patent)
.75 pound crystal 80L
.5 pound pale chocolate malt
1 pound lactose
Mash at more like 151/152
For my water for stouts, I use tap water- the one good use I have for my tap water! Starting with RO and building up is probably a good way to go, but I really can't help with that very much at all. (The Brew Science forum area probably can help you more with building water).
With that much roast, I'd target IBUs of 25-30 depending on OG,
Broken Leg Brewery
Giving beer a leg to stand on since 2006