Hi hfein 1, I'm not sure if there is a place in Beersmith where you can manually change the water to grain ratio of your mash.
I usually just take my grain amount and multiply it by the ratio of water I'm wanting to use. Example 15lbs of grain times 1.5 = 22.5 quarts of water.
Or if I wanted a thicker mash then I would use 15 lbs of grain times 1.2 =18 quarts of water.
Hope this helps.
stevemwazup