My numbers until someone decided to help me kill the mob...

Test subject: Paladin Lv60
Test target: Clockwork Harvestman Lv59
Gear parry: 530
Gear block: 322

Swings: 2378
Misses: 312 (13.12%)
Hit: 2066 (86.88%)
Block: 457 (22.12%)
Parry: 256 (12.39%)

Taking the base parry rate of 5.865% for 354 parry, that's a 6.52% increase in parry rate for 176 increase in parry rating, giving about 27 parry rating per 1% of parry.