The hundredPercentBase property defines the range to be used as a base to compute 100% engine health.
By default this is SAFE_RANGE which only displays positive percentages while the engine health is in the range, where it will never fail. Negative values denote risk (engine could fail).
If you don't want the value to become negative at any time, set this to MAX_RANGE.
This property won't have any effect, if your engine doesn't have any variance in lifetime.