A good tip when masking a design like that - lay your base colour (white) as you described, attach your masking (decal in your case) then spray another coat of the logo colour (white) before laying down your background colour (black). The coat of logo colour after masking, in my experience, takes care of any bleed under the edges of the masking and seals it up so the other colour doesn't bleed and you get nice crisp edges. Not done this with decals, but don't see why it would be any different.
