Unify storage calculations.

Due to one calculation using longs and the other using doubles, there
were situations where the percentage was off-by-one between the two
places.

By changing both places to now use doubles to calculate percentages, the
numbers should be unified.

Change-Id: I4f4ed19326562e198273adc60a1593c7d1a6b6a7
Fixes: 37923610
Test: Manual