Q: How much movement do the different levels of boots add?
Level 1 adds 50, Level 2 adds 70, Level 3 adds 90, etc.
Q: How do movement modifiers work?
First, you have your base movement of say 300. You add boots first, always boots first, so if you have boots added.
Let's say you get boots 2. That's 370 movement. Then, you get 35% movement speed buff from items or runes or masteries or buffs: that 370 is multiplied by 1.35, or 499.5 movement speed. If this is the only thing affecting your movement speed, the diminishing returns formula kicks in.
Now, let's say you get slowed by 38% (ashe arrows). What happens is the ORIGINAL 499.5 is multiplied by .62, and you get 309.69 movement speed. Because 309 is not above or below diminishing returns, you stay at that speed.
Q: What are these diminishing movement returns?
For each point of movement you would have above 490, that amount is reduced by 50%. So if your final movement speed was 748, it would be reduced to 490 + (748-490)*0.5 = 604. For each point of movement you would have above 415, it is reduced by 20%. So if your speed after all mods was 445, you would end with 415 + 30 * 0.8 = 439.