Jan,
Sheep do need copper. They just need a small amount of it. I have not seen copper shown as a percent, usually it is in parts per million ppm.
If your mineral is formulated specifically for sheep you are probably good if not I wouldn't use it.
Here is a link about copper for sheep.
http://www.merricks.com/tech_copper_req.html
.3457% is equal to 3457ppm which would kill a cow.
Look at your label again and see if it says 0.3457 ppm? They usually switch from % to ppm as you go down the mineral ingredients.