Yes, creating a site using 'dual' mode is the way to go as it allows you to create a desktop and mobile site within the same project. The html file is the same for both. It's the CSS code that determines whether you see the desktop version or mobile.
There is no right or wrong answer in what settings you should use but I would recommend no less than 1366px for desktop and around 480px for mobile.
When in dual mode, you will create your desktop layout and on the mobile layout you simply re-arrange the same objects from the desktop to fit the mobile layout.
However, I will caution that if you never built anything in dual mode to take your time and baby steps. Start with your header and then arrange the header for mobile and work your way down.
I will say though that SS Pro is not ideal in creating a truly mobile experience due to the fact it only allows for 2 layouts. As you can see, there is a lot of space between 1366px and 480px. Back in the day, when you just had 2 sizes: computer desktop and mobile phones it was ok But now you have so many size phones, desktops, laptops and tablets. I've used SS Pro many times in the past but haven't for the past 3+ years. There may be other software better suited to create your site. Email me if you are interested.