Hi, Joe.
Lkmuller has it exactly right. As the human voice is generally limited to one and a half octaves, melodies do the same. For numbers above the main octave of the song, I use 9, 10, 11, etc. ; for numbers below, I use bold/italic font.
You can even indicate note duration: for eighth notes, one underline; for sixteenth, two; dotted quarters get one dot; half notes, two dots; whole notes, three dots.
Have fun!


Cheers! -Alastair