Binary. You can get up to 1023 on your fingers then and it's pretty easy. I used to do it when I was a photocopy monkey.
You missed a trick there, let's face it when you count, you don't start at Zero as there's nowt to count ... you coulda denoted no fingers as 1024 
Just think of all those copies that you coulda have done extra, lol.
Oh, and I still see a billion as a million million too....