If that's "AFK scenario" stagger = 20%, as you don't have shuffle up. If it isn't then average SotR uptime for paladins.
First thing first, stagger != reduction.
In fact with no PB usage mastery does nothing
atm i'm at 10k mastery for 15% more stagger (accounting for 5% base) so ~5k mastery give 10% more stagger, not 30%.
And again stagger doesn't magically vanish, we take the full hit, part as a hit, part as a dot, but the FULL hit, unless we PB.