Ravi writes :

> But sometimes the melody will start around another note.
> For example, one measure of the song will have a G chord harmony with
> an ascending melody phrase: B C D E F.
>
> In this case, it may be useful to think of it (for at least one
> measure) as B Locrian.

I don't think it'd be useful to do this. Your melody is in C.
What's the point of changing modes for every phrase in your
melody ? The above phrase is just a run of notes, probably
leading somewhere, which isn't shown. If the F is still part
of the G chord sound (and not part of the next chord) then you
can think of it as providing a G7 sound. But it's probably just
fill.

For example over a C chord, you could have an E leaping to G or
you could get there by stopping on F along the way for a smoother
ride. You aren't changing modes, the F is just fill.

Paul P