There is no app or spreadsheet out there that will tell you how to mash your grains. This is very process and recipe specific to get the results that you want.
If you are looking to brew an APA, then look at the guidelines for that style. This will give you an idea of the starting and finishing gravity ranges to shoot for. Customize your equipment profile to match your set up, enter in your ingredients to match your recipe. Now set the style for APA. If the OG is high or low, you can click on the slider for OG and set it to where you want it to be and the program will adjust the grains accordingly. You can select from the different mash profiles (or create your own) seeing the effect each has on your OG and FG. Stick with the one that seems to match what you want.
Since this is your first all-grain, don't sweat getting the numbers right on and concentrate on taking the measurements to fill out the blanks on the 'session' tab. The program will calculate your actual efficiency in the end and you can update your equipment profile with the actual numbers for losses, boil off, and efficiency for the next brew.
Likewise, the FG is a function of the yeast strain, grain bill, mash temperature, fermentation temperatures so don't sweat that much either. You will eventually learn what mash temperatures will get you the ending gravity you want for a given grain bill and yeast strain and can adjust those parameters to get the software to better predict your results.