About 10% a cycle yes. I think most feel 10-20% a day is good but more doesn't hurt, its just a bit more wasteful. My nutrients are dirt cheap so for me I don't mind.
Yes thats to prevent nutrient buildup, ratio imbalances and keep a stable PH. This is my forst coco grow but I would say if your not getting runoff with inorganic nutrients its only a matter of time before you have issues. This is mostly seen early to mid flower when it catches up to growers. Its kinda deceiving because everything looks fine and ppl say I only feed this often or I don't need runoff then all if a sudden everything is not fine.
In coco if you watch the runoff its gives you a good idea of what's going on in the rootzone. Maybe not exactly but close enough to see which way its trending.