It has been a long time ago when I first started playing numbers but I think I remember I tried dowsing with needle and thread. I just wrote down the numbers in a circle and hung the needle in the center. It did not work the first few tries so I gave up on it.
I personally do not think it is black magic.
As another personal experience on dowsing I purchased a piece of property to build a house on. It needed a well drilled. So I knew this friend of mine who was a dowser for water. He came and dowsed the site and I watched him dowse the lot and he came to a spot and the dowsing rod started to quiver in his hands and the fork of the dowsing rod pointed directly to the ground. He said thats it and it is a strong vein of water and not too far down. The well driller came and I had him drill there at the designated spot.
The driller went down 68 feet and hit 10 gallons per minute.
If you want more info on dowsing just google dowsing.
I found this site that had a good read on dowsing.
http://paranormal.about.com/library/weekly/aa072902a.htm
As far as other means of doing numbers when I am looking at my worksheet I will have numbers come to my mind when looking over the sheet.
I think they call that scrying.