If you need to write out the song in numerics, write roman numerals with chord values (II-7, V7, I, etc) relative to the key of the moment and then use numbers (R, 1, 9, 3, 11, 5, 13, 7) to represent the melody notes.
So D-7 / G7 / CMaj7 becomes II-7 / V7 / IMaj7
And minims: F, A / G, B / C, B becomes 3, 5 / R, 3 / R, 7 over those chords
Although the actual chords and the actual notes might be a better way to go, using standard notation.
Watermelon Man is just a blues in F, it contains 3 chords I7 (F7) and IV7 (Bb7) and a V7 (C7).
F7 is the tonic chord which happens to be the dominant from the key of Bb, but that's not relevant in terms of the functional harmony. It's mainly just uselful for finding your way around the instrument.
My 2ps anyway
