...Windows 8 used 15.3 GB and left 34.6 GB of available space. But, it is importatnt to keep in mind that I choose 50GB purely for illustrative purposes.

Also keep in mind that in Windows 7's Disk Manager you can also extend a partition.

So If you needed more space for installing other apps in Windows 8, you can always shrink the Windows 7 partition again and then extend the Windows 8 partition into the newly created free space.