Am i looking at it the wrong way? but did you put the decal on the TOP side of the ramp?
I would stick it underneath the ramp, to protect it from ball-wear.

Also, if it is stuck on the underside, you can cut the 3M tape a bit wider, so the side edges align at the edges of the ramp to help conceal the decal-edge.

Hopefully you scanned the decal into your PC for future use, since you can't get them ;-)

my 2c ;-)
MM

Definately underneath, you can not cut the tape wider, as the tape is only really gum, the tape backing just supports the gum whilst you are laying your decal down on it, once you remove the excess gum with your finger the decal is the same as the original with new 'gum' adhesive, then you just peel off the tape backing and apply.