The correct power is -8 since it takes shifting the decimal 8 times to the right to get the correct number which makes the power of the 10 -8.
I assumed that the original number is mimihimes' post was 0000002509 meant 0.000002509 which can indeed be written as 2.509 x 10^(-6).
If instead 0000002509 was meant to be 0.0000002509 then if I'm not miscounting zeros then it can be written as 2.509 x 10^(-7).
Either way, both compactings of the respective numbers should be correct ^^.