It takes a bit of getting used to for ships. Headus (the designer) is open functionality requests. He's added a lot of "snap" type functionality for mapping mechanical designs. If you use it and think that it'd be better if it could do x, y, or z, just tell him (Be sure to explain why you need it.) and typically he'll add it.
I've always had a problem with even pixel density while mapping. It's automatic with this though.