This is pretty identical to how armor diminishing returns works. Armor is a straight translation to a % value UNTIL you hit the flex point, somewhere around 60%. After that value you get less % protection per point of armor. I'm pretty sure MasterCosmo graphed it out at some point. 12000 armor does not equal 120% protection since we start the diminish effect at 60%.
For the most part no one can go above 20% with attachments and power ups with dodge, parry, block chance, dodge, evasion chance and strike through chance. We however put all of those stats on the diminishing returns forumla as a safeguard against future stacking issues.
Just like armor if you breach the flex point, you get less amount of that stat per point (this is when it makes sense to balance your attachments rather than gearing for just 1 stat or "stacking". So if you find a way to get to 25% block THROUGH attachments and pups it would fall to 21% after it diminishes. Any value a buff adds/expertise adds would be added on AFTER the diminish. So a medic buff for 10% block would add ontop of the 21% and be a 31% block chance.
Jedi get most of their parry % from Saber Block expertises which add % parry and it adds after the diminish, so their numbers will change very little.