A few points.
Originally Posted by Jorge
I use reflected readings for calculating N values but only incident readings for calculating SBR values. Calculating SBR values from reflected readings or N values from incident readings might work with some subjects but with most scenes it would involve some type of conversion factor that would make exposure determination very complicated and prone to misinterpretation. Which is one of the reasons I use the Expo/Dev program for figuring exposures -- you choose either the SBR or N system and the program does the rest.
About the SBR of 4, there is no such thing. All subjects that are evenly illuminated (assuming no glare or flare) will fit into a five-stop luminance range, which is two stops less than the normal SBR of 7. Thus, for all practical considerations an SBR of less than five is impossible. Even a real five is highly unusual as in even the flattest of lighting situations I rarely encounter scenes with SBRs of less than 5.5.
Finally, one of the dangers in working with either the N system of SBR sysem is misinterpretation which can lead to an exaggeration of the difference in luminance in the scene and to inflated values, both high and low. A typical example with the SBR system, for example, would be in an outdoor scene in full sun but with some deep shadwos( as in say a recess in a boulder). If you were to meter the deepest shadows of the recess your readings would provide a SBR that is greater than it should be and this will cause you to both over-expose and under-develop your film.