Wow, that is a lot of roast. But I'm assuming this must be for more than a 5 gal batch!
No, you wouldn't need or want to modify the water/grist ratio. Remember, you will eventually be adding those roast grains back to the mash and there are minor benefits in having a slightly thinner mash. Additionally, the pH prediction for that roast-less mash is based on the water volume you have originally input.
The mash pH prediction is a balance that looks at the total acidity of your mash grist and the alkalinity of the water you are mashing with.