Thanks for the replies and links. All the different explanations on how surface brightness is the true indication of contrast make perfect sense. It helps paint a clearer picture on how the numbers work. I like it!!!
On the practical side. I use a 35mm, which gives me around 60x. I start a low mag to try to spot things easier. I am going to experiment with different magnifications to see if I have better luck with M51. It sounds like it should be visible at the right place in the sky and maybe I have not learned to spot what it looks like in real life.
A couple thousand years ago, Ptolemy decided to divide the stars in the sky into "bins" of magnitudes. The brightest stars he called "First magnitude" because they were of primary importance.
The faintest stars he saw with his eyes were binned into the 6th magnitude and others in between.
When instruments started measuring these brightnesses, we discovered that First magnitude covered a wide range (1+ to -1.42) and that there was approximately a ratio of 100:1 between the faintest and brightest.
Since our eye sees brightness logarithmically, that meant the 5 magnitude spread was 100, so each magnitude represented the 5th root of 100, or 2.512, i.e. 5th magnitude is 2.512x as bright as 6th magnitude, 4th magnitude is 2.512x as bright as magnitude 5, and so on.
On that same scale, the sun is magnitude -26.7, the full Moon is about magnitude -12.5, and Vega is magnitude 0.03 (pretty close to mag. 0).
So, magnitude 10 would be 2.512 to the tenth power dimmer than magnitude 0, or 1/10000 times as bright. By calculation, the brightest star I've looked at through my scope is about 37 million times
brighter than the faintest star I've seen. Quite a range.
Surface brightness is calculated from the size of the object, so a magnitude 10 galaxy 1' across would appear quite bright, while a 10th magnitude galaxy 15' across would appear quite dim.
So you need to not only know the surface brightness, but also the size and total integrated magnitude figure to get an idea if the object will be hard to see or not.
And, galaxies in particular have cores that are brighter than the outskirts. You might not see spiral arms, but the cores of very faint galaxies are usually visible.
Your technique of starting at low power, then bumping it up to view the object is a sound one, though this might be hard with planetary nebulae, which are usually small. With those, starting at a medium power might aid in finding the objects.
As for M51, try a magnification around 15x/inch of aperture to see it well. Lower powers will show the galaxy fine, but you'll not see much detail.