I have a keyboard, a Roland RS-50, and I have been playing around in D Double Harmonic major a lot recently. That Wikipedia article says that you can play the scale with quarter tones, and I'd like to try this as I've never played with quarter tones before. On page 58 of the keyboard owner's manual (linked above) it says you can individually tune any note of the scale up by 63 and down by 64.
My problem is this: tuning down D# by the full 64 doesn't make it reach D (in equal temperament tuning in which all notes are set to 0) but it seems to get quite close.
How do I tune it to exactly 1 quarter tone down? What are the degrees in this tuning setting?