Like to answer but you don’t say what key the piano music is in, You just say “key -6” so I don’t know what to base this on.
It’s very confusing the way you explain it.
can you post the music sheet so I can see it?
The intervals you mentioned are right. What I don’t understand is why you’re coming from C down to F#?
Is your piano players music in C?
If so, is he transposing it down to F# for the singer or what?