PCTINCREASE determines how fast the size of the next extent to be added gets increased. So in your example, the initial extent would be 10M, the second would be 10M, the third would be 30M, the fourth 90M, etc...
Again, I'm not sure what your looking for, but that's how what you wrote would be interpreted.
Oh gotcha. So if I do:
STORAGE (INITIAL 10M NEXT 10M PCTINCREASE 0)
The initial extent is 10M, second 20M, third 40M, fourth 80M ??
I want to double each time.