The mash guidelines section is the place to provide temperatures and times for the mash process. Volumes are optional. For all grain recipes, an optional starting mash thickness field appears (normally between 1.0-2.0 qt/lb or 2.0-4.0 L/kg).
Given the wide variation in mash processes, equipment, yeast strains, and fermentation temperatures, the mash temperatures do not factor into the estimated FG of the recipe. What you can to do adjust FG is change the yeast attenuation.
The Brew feature is where mash thickness, infusion temperatures, and water volumes are tracked.
Here is our approach to mashing:
On the recipe page, enter temperatures, times, and rough amounts. The fields are all optional. From there, save the recipe, then click the 'brew' button. The system then uses your equipment profile to figure out exactly how much water your will need for the mash. (You can setup your equipment profile under the gear icon in the upper right.) Then go into the brew feature's Mash Calculator tab. From there you can adjust the mash ratio, grain temp, mash temp, and do infusions, etc. It saves every detail along the way. It also helps zero in on the target water volume for the brew.