Magnitude, Surface Brightness, and Size?

I have been looking over lists of astronomical targets (especially clusters, nebulae, and galaxies... and some double and variable stars too). One thing I have noticed is that there are many targets (namely, clusters) that have either the same magnitude, brightness, or size (while the other two factors are different), yet, one I can see and the other I can't. Is there some kind of equation (or algorithm) that I can use, based on magnitude, brightness, and size, to determine which targets I am likely to see through my scope, before I try hunting them down?