I would have said leave it blank, but I was reading something recently where they had claimed that it had to be filled in. I think it was in a "Lost Cousins" newsletter.
Yes, found it:
http://lostcousins.com/newsletters/l...news.htm#Wills
I don't know of anywhere else to find the folio number if it isn't on the calendar, and I have never given a folio number when ordering copies in the post. So hopefully you don't need it - but I would hate to get the blame if you lose money by ordering without it!