When the solo part is written in the highest manual part, it will be played with the right hand on its own manual. This is very common in organ music. We would consider this a soprano- or alto-range melody. Next, decide the range in which it should sound--its pitch foundation.
If you expect your solo to sound in that same soprano or alto range, base your solo stop or combination on an 8' pitch foundation.
If you want to change the sounding range of a soprano melody to the tenor, or an alto melody to the bass, you might build on a 16' pitch foundation.
If you wnat your soprano melody to sound in a very high range, or an alto melody in a high soprano range, build on a 4' foundation.