Sounds like you need to experiment with camera angles when you're placing your decals. What I would suggest is to manipulate the view using the camera until you get to an angle where the decal will either project a straight line or avoid bleeding over the edges. Then change the projection mode to camera (not body shape) and you'll find the decals warp according to your camera angle. I use this a lot when a straight decal needs to bend, for example on the nose of a car, by adjusting the angle of the view until the panel looks like it's "flat" in the view, then changing the projection mode. You'd then need to adjust the proportion of the decal as it stretches, and may need to change the depth, but I find this is the most accurate way to achieve what you're after.
Does that make sense??
