Well it depends on what version of Silent Night you want to code. You could have the spaces between sounds be longer or shorter, just whatever sounds best to you.
Because we have no control over the audible duration of the (xylophone?) tones, I think you'll find that the "instrument" is going to dictate the tempo, just based on what sounds good to you. For most of what I have tried to do, the tones are just... too... long. But for the tune you have chosen, they might be just right.
Hi! I like to use 500 for a quarter note, 250 for an eight note, 1000 for a half note, and so on.
125 for 16ths, 62.5 for 32nds, 31.25 for 64ths?