How much were various medieval taxes like the scutage on a knights fee? Like in terms of percentages (or were taxes fixed amounts not percentages?)?

Some taxes were “fixed” by tradition – so for example, at various times scutage was one mark (2/3 of a pound), one pound, or two marks. King John got himself into trouble by demanding first two and then three marks. On the other hand, starting in 1166, there was a tax on moveable property and income which was known as the “tenth” that was a percentage tax (10%, natch).

Generally speaking, over time, there was a movement by monarchs away from fixed taxes – which didn’t keep up with inflation or economic growth – and towards percentage taxes, which did.