I have my first ever sheet cake, and I'm running into some issues that are probably very simple, but I am not finding the answers I have been searching for.

I just need to make a 2-layer 12"x18". Yellow cake, filled with fresh strawberries and vanilla buttercream frosting throughout the whole thing. It seems I have everything figured out, except for how much batter to make and how much of the frosting to make. I am also making a dozen cupcakes with it (same style as the cake), and I think that is what is throwing me off. I also do not use box mixes.

Any help/answers/tips would be greatly appreciated! I've been reading a lot of forums and charts, and it's putting me into a tizzy!  

A 1x recipe for the cake batter I am making makes 50.74 ounces. The buttercream makes 33.6 ounces.