I'm hoping to understand the impact of light pollution on surface brightness. Also, does surface brightness of OC somehow differ from that of a PN or GC? That LP brightens the sky and must limit views of DSOs from an urban setting is clear but is there a formula to estimate the limit of what I can see?
Specifically: Using 15x70 Oberwerk DeLuxe, supported, I am in Chicago but observe to the east over the lake so my guess is Bortle 8. I only go after DSOs on nights when the transparency is very good. I have seen M37, an OC with a surface brightness of 12.8 but have not been able to see neither M27, a PN with a surface brightness of 12.3 nor M78, a reflection nebula with a surface brightness of 12.6- despite multiple attempts.
Here are some thoughts. First of all, take all astronomical data with many grains of salt. In particular, it is hard both to define and to measure the brightness of nebulae. That's due to many factors, the two main ones being they typically have uncertain borders and the fact that they typically shine in only a few specific wavelengths rather than being broad-spectrum sources like stars.
Open clusters also have two different aspects -- they may be detectable either as diffuse glows or as individual stars. My guess is that using 15x70 binoculars in Chicago, you detected M37 only as a diffuse glow; is that right? In that case, the surface-brightness figure is at least approximately relevant. When you're detecting a cluster as a collection of individual stars, other criteria apply.
For the record, M27 is generally fairly easy to detect with small instruments under bright skies, whereas M78 is quite hard. That is due not only to surface brightness but also to size; given two objects with identical surface-brightness profiles, the bigger one will always be easier to detect. Often a lot easier. To phrase exactly the same thing in different words, given two objects with identical surface-brightness profile, the one with the greater total brightness (lower apparent magnitude) is generally much easier to detect.
Another factor is that you're observing to the east, when objects are far from their highest in the sky. That doesn't matter much with M37, a far-northern object that's already quite high even when it's due east. It matters a lot more with M27, which is just 20 degrees north of the celestial equator. That means that it's quite high when it's due south, but still down low in the light-pollution haze when it's rising in the eastern part of the sky. Yes, that light-pollution haze can be quite strong even directly opposite the major light sources.
There absolutely is and can be no formula that can predict what you will see, but you'll get a good sense of it over time. For the Messier objects in particular, you might want to look at the ratings in my Urban/Suburban Messier Guide.