I'm not sure what you don't understand, so I will explain in detail how your block chance is calculated.
The base block chance is what's displayed on your shield when hovering the mouse over it. If the shield has a blocking bonus itself, it's already included. Then add the other blocking bonuses to it, if you have any. Subtract 15 from your dex score and multiply that with the blocking value, then divide that number by your level*2. That's your block chance. If the result is greater than 75%, it's 75% instead.