If its just 1 note then go for middle C, though it could sound a bit rough as you go up and down the full scale.
If you have the option of doing this in Software first you could experiment and see how that 1 note works up and down the keyboard.
Enjoy your spare HP, don't rush to fill every last space, this is not like filling sticker books. Resist the urge to 'complete' your rack, its never complete so just relax.