You cannot point your gossen and read the light and dark places and come up with a SBR. You are measuring different things, the reflective readings are measuring luminance of the subject, IOW the light they reflect. The incident metering system relies in measuring the illuminance or light falling on the objects to be able to determine the difference in light strenght (contrast) and expose and develop the film accordingly. Since the incedent meter averages the light falling on the object, it is a more reliable way to expose for the shadows, in addition if you want increase or decrease the contrast of the negative, all you have to do is give a little less or more light to the meter to determine the exposure. All the light meter reading does is determine the contrast range and the required development times for that specific contrast range.

I have found this system far more reliable than the ZS or spot metering. But you do have to do the inital testing, after that, it is a piece of cake.