Well, you'll see the red (bad) and blue (good) sections that the roll will randomly end up in. Blue is an upgrade, red is a downgrade. So anything blue is good, and the further to the right into the blue, the better. The best possible roll is at the very right end of the blue section.
edit: also, be aware that the red/blue sections are shown according to your current stats. So you'll see that after you accept an upgrade, all future upgrades will have a bigger red section and a smaller red, as your current FSD is already better than the norm, so there's only that much more left for you to improve. It makes it easy to compare though, as it means that even when tuning an already upgraded module, the blue section is still the one indicating that the roll is better than what you currently have.