Figured it out.
${Target.AggroHolder.ID} != ${Me.ID} || ( ${Target.SecondaryAggroPlayer.ID} == ${Me.ID} && ${Target.SecondaryPctAggro} < 75 )
My casters use this with a command:/sit at the end of thier dps entries, works wonderful
I also do a stand check on my aggro entry to stand them back up if they pull aggro by sitting.