The first 5 are fairly obvious
0 = Closed Fist
1 = Thumb
2 = Th + Index
3 = Th + Index + middle
4 = Th + index + middle + ring
5 = Th + index + middle + ring + pinky

the sequence continues the opposite direction by changing the next digit in sequence.

6 = Th + index + middle + ____ + pinky
7 = Th + index + ______ + ____ + pinky
8 = Th + _____ + ______ + ____ + pinky (Hang loose)
9 = __ + _____ + ______ + ____ + pinky

then keep going

10= __ + index + ______ + ____ + pinky (Metal Rules)
11= __ + index + middle + ____ + pinky
12= __ + index + middle + ring + pinky
13= __ + index + middle + ring + _____
14= __ + index + middle + ____ + _____
15= __ + index + ______ + ____ + _____

Two hands does 256.
And, it is viewable from a distance unlike counting the joints on each finger.