If your engagement ring has a flat surface underneath the gallery like the ring shown, a flat band will fit perfectly flush with it.
(Meaning the band will sit closely to the ring with no gap).
Make sure that the band's concave matches the ring's convex.
Like the example: in your rings convex is 3mm, you will want to make sure that the band’s concave is 3mm. Otherwise, there will be a gap, meaning they will not be flush.
Your band can completely transform the look of your ring, so get creative with your combinations, try different styles, and see what fits well for you!
And who said you can only have one band for your ring? Have different styles to fit your different moods!