[Improvement]Fix loss of permissibleRatio calcaulation accuracy.

The previous method performs exponential calculation after natural logarithm and subtraction operation, which brings loss of calculation accuracy. So it is recommended to modify this method from the perspective of scientific calculation.

Change-Id: Id6c7ca3c222226d642bd075519da693a8538dba6
Signed-off-by: linkai <linkai@xiaomi.com>