When I’m looking for the best images, I will decide on a “fixed” boat speed (perhaps just above idle, 2.5-3 mph) and then tweak two parameters on the graph: (1) the side-scan range and (2) the chart speed. I’ll adjust those while I’m driving along, monitoring the images the whole time, for the best possible images.
With a small amount of time invested (and I’m talking way less than 10 minutes), I know that I will have dynamite images at that particular boat speed. If I need to be moving slower or faster (like if I’m pulling 3-ways on the river or power trolling), then I’ll need to go through the image optimization process again for the best images at the new speed. The trick is to write down your settings in your fishing log, and you can quickly reset to the best settings needed for any particular speed that you are moving.
Having said that, the factory defaults are pretty darn good, so you shouldn’t be intimidated by the small tweaks I’m talking about. Get comfy with your Humminbird, start to wow yourself and your buddies with great side images, and then start to fiddle with the settings for the best images possible!