I have that same unit, I believe. 598 ci HD, with "side imaging" labeled on the card door. The DI is a blended view from the SI, not a true DI. The screen is relatively small for viewing side images. I found I was best served with keeping the SI distance to less than 50 feet per side. I hardly ever use the DI view for my fishing but I have it set on the narrow or medium width, usually narrow. As to setting, I'd start in the middle with both the sensitivity and contrast in the SI. DI I usually ran a higher sensitivity for detail. In SI, if your bottom is muddy, you can go up on the sensitivity a fair amount.
When viewing the SI, the reflections of crappie size fish are very small and are easily missed in real time, especially if you are viewing 50'+ per side. This was my first GPS unit and I like that capability very much; also my first unit with a card reader to make recordings of my actual trips. I learned a lot by reviewing my recordings at home and changing settings while doing so. Much more relevant than the simulator as this is YOUR water and YOUR conditions.
I thought this was a good unit but I wanted a larger SI unit for my type of fishing but was well served with this one while it was in use.
On Humminbirds website, they probably have a link to the owners manual if you didn't get one with it. It's very thorough and helpful.