How many percentage points do you gain for every point specced into Parry?

Base = 5 % Skill Bonus = 0.5 * (parry spec + realm rank bonus + template bonus) %
Dex Bonus = 1 / 40 * ((dex * 2) - 100)% Myth Bonus = Mythical parry %
Realm Ability = Mastery of Parrying %
Total = Base + Skill Bonus + Dex Bonus + Myth Bonus + Realm Ability

We can get rid of Myth Bonus here.

Base = 5 %
Skill Bonus = 0.5 * (parry spec + realm rank bonus + template bonus) %
Dex Bonus = 1 / 40 * ((dex * 2) - 100)%
Realm Ability = Mastery of Parrying %
Total = Base + Skill Bonus + Dex Bonus + Realm Ability

Realm rank 1L0 with no MoParry.
20 parry
0.5*(20+0+11) % = 15.5 %
40 parry
0.5*(40+0+11) % = 25.5 %

10% difference to parry with 20 vs 40 skill.
This of course doesn’t include the other bonuses. Add 5% for base chance as well.