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Clicking on the Goal button in the upper left of the fermentables box you enter your desired Original Gravity or ABV and click submit. Your fermentables will change to percentages and the amount of grain will be automatically calculated.
Set a goal for Original Gravity of 1.050
Now select a fermentable from the dropdown and enter what percentage of your grain bill you want. In this case 90% of Pale 2-Row will get our gravity to 1.045 and require 9.63 lbs. Next we'll add another 10% of another grain to hit the goal of 1.050.
Entering 10% (or clicking the % link next to the empty box will auto fill the remaining percentage to get us to 100%). Choosing Crystal 20 tells us we need 1.13 lbs and will update the Original Gravity in the stats bar automatically.
To switch back to entering amounts instead of percentages, click on the grey amount input.