iOS exposure algoritm

I need implement camera exposure change for video recording. I look for default autoexposure:

_device.exposureMode = AVCaptureExposureMode.ContinuousAutoExposure

It works fine. But I need more complicated logic with exposure and wide range of Exposure time change [0.01, 0.8]. Default algorithm have [0.01, 0.25] of Exposure time range. So I try to repeat apple algorithm. I know when image might be lighter or darker, but I need implement some logic for ISO and Exposure time change. I already implement my algorithm but it works not how I need.

Some result of experiments with standard algorithm (I capture standard exposure change when environment light change; exposure scaled for normal display on chart):