Here is an important lesson I learned about using floating row cover with raised beds. DON’T SKIMP! You have to have floating row cover that is wide enough to cover the width of the bed so that it can reach 3-4′ high and still have enough left so that you can anchor the sides to the ground (mine is 10′ wide and my beds are 4′ wide so this works). Then you have to cut it long enough so that you can gather the ends and weight them down. You do not want any gaps. So you would do at least 24′ for a 16′ long bed.
The problem is that on the one hand you don’t want to waste expensive row cover, even if you buy it in the more cost-effective commercial quantities. But, if you skimp and insects get under the cover, they are in heaven while you are assuming that your plants are safe. When you take off the row cover, you will be in for a rude awakening.
So … do not be penny wise and a pound foolish … use enough to do it right. I learned the hard way.