Wheel arches are a bit tricky to get the line nice and clean, but here's a little trick I do, run a line of masking tape around the edge of the wheelarch, and then you have a hard edge to cut to, and it also stops the blade going off line, put the tape on, put on the BMF, trim, following the line of the masking tape, and then remove the excess BMF and the masking tape, and you end up with a nice clean line ....
If you use thinner masking tape, you can do it in one piece, instead of using numerous pieces of thicker tape