Get out there and play. You will learn the percentage ranges best best by simply playing many times. There is a lot of improvisation with characters given stage layout, percentage, character properties, and directional influence. Only a few situations I can think of are worth memorizing for Marth's throws.
On that same note Marth's throws are good for the control you get over your opponent. For example, you probably will find it best to try keeping a person locked into a bad position for as long as possible. Doing your Uthrow -> Dair on Fox for example is probably going to release Fox from a bad position in most cases. Ultimately, doing combo's mean nothing since a person can die at 0% if something dumb happens. Your goal is to get your opponent into a blast zone. Throw's help to start your chain of events to put the person into that blast zone.
Without getting into a long analysis of an example let me just finish by saying that you should make choices with your throws that further this goal of keeping your opponent in a bad spot for either as long as you can or better yet until they get KO'd. Try to avoid simply throwing away a person for them to instantly get back into a neutral setting.