I've found that using the 2/3 measuring cup helps keep the guess work out of evenly-scooping and eliminates unnecessary mess. This fills them between 2/3 and 3/4 full, giving a nice dome shaped muffin in the end. Experiment, and see what works for you!
When adding topping, the tablespoon measure unit works great for these muffins. Again, less guesswork and less mess!