I think a much simpler fix would be to either crank up the enmity generated by Flash or give Circle of Scorn to PLD earlier than 50. PLD's biggest weakness sub-40 for many people is a lack of true AoE threat like WAR's Overpower. Changing Shield Oath to 30 is only a band-aid fix. If a PLD is going to have trouble holding enmity, it'll be AoE and not ST. Shield Oath will only make it harder to lose AoE enmity, but a PLD that isn't paying attention will still lose it.