I've run into this as well, but in a different way. It drives me crazy. I usually set my water by absolute amount, instead of by ratio. For example, if BeerSmith says I need 20.34 quarts of total water, I always sparge with 12, so I will set the mash water to 8.34 so I only have to set aside exactly 12 for the sparge and dump the rest into the mash. What happens is that BeerSmith calculates the ratio, then rounds it to one 1.5, and I end up with something like 8.532 quarts.
My solution is the same one I use whenever BeerSmith (1 or 2) has rounding problems. Type out the number into the field and hit enter without tabbing out or clicking anywhere else.
In more detail:
1. Click on the water amount field
2. Edit it with whatever long number you want, 8.325 for example
3. Press enter
It will save your more precise value, but if you open your mash profile and edit something it will round again, so watch for that.