To be Christmassy, break it into a red sequence and a green sequence:
8,2,4,1,2,0.5,1,__
The red numbers go:
8, take half, 4, take half, 2, take half, 1.
The green numbers go:
2, take half, 1, take half, 0.5, take half, __.
So half of 0.5 is 0.25.
Edwin