Hi Nige,
Place the stickers centrally on the back of the target so you shoot through them.
This idea works well as long as you are close enough to the black bit in the middle when you shoot the card!
You could try putting the stickers centrally on the front of the target ........but it makes it tricky to score
I guess with a self scoring ( trust system) competition it makes very little difference where you put the sticker (within reason) but knowing it is fixed to the target is often worth - 2 points (or more) so having it on makes it a "pucka" match card
Welcome to the MPL