As the originator of the correspondence, it is entirely your choice as to what file number you use. If it's a personal request, then as Occam has said, your service number is appropriate. You do not have to follow the CF filing system. Think of it this way, the file number is so that YOU can find it in YOUR filing cabinet. When your OR logs it in, they will place it in the appropriate file according to THEIR (i.e. the CF's) filing system. A written response will then have the appropriate CF filing number (which you could use for future commentary on the subject, but that's entirely up to you).
If I was in your position, I would use my service number. It's the easiest option.