I would say, yes to drawing a line between bottom of pedal rotation, and bottom of tire, and if you block that, it should keep everything dry for the most part. I'd caution against going too low with something fixed too firmly, because if you get a flat, it will scrape and possibly push the whole fender up, potentially damaging it. Even if fender is OK, you could damage your mud flap and ruin its mounting points.

- Andy