Couple suggestions if you ever try to do wheel wells like this again in the future.
First, tape the plastic to a board or your tabletop so it cannot move around.
Second draw a long straight line on the plastic card to make the center line for both circles.
Third draw your circles using the same centerline described in the second step. Poke the center of your compass into the line and the draw one circle, then do the same on the same line for the second one.
Lastly try not to press so hard with the lead of your compass and pencil, I think you just need a faint line. This will keep your pencil from loosing it's point so fast and thus getting a thick line.
Just my obeservations, I'm no expert on the subject by any means.
Hope that helps and makes some sense to you. Good luck, you doing a great job thus far.