I have Wolf Gauntlets that increase my block by 18% and a shield that increases it by 13%.

Assuming I use them cooperatively, does that mean I block 31% more damage with my shield?

What's the highest amount of percentage I could block?

It stacks but there are several factors involved. See this thread (post #4) for a good explanation.

http://www.gamesas.com/topic/1342463-complete-character-design-freedom-damage-resist-caps-and-ridiculous-damage-thread-10/

Thank you.