I just say at the beginning of an instance, Skull is first target, cross is the second target, I'll call for CC and mark it, if its necessary, otherwise, please don't. No need to make it more complicated then that.
One thing though, its good to be consistent with your markings, in single instances, and across your WoWing in general. There are general conventions for most of them, but no set thing, but its good to have your own personal set that you always use. For example, with me, Skull is dps, Cross is always the next target (not off-tank), moon is always sheep, star is always sap. Pink diamond is usually a market for myself, so the group can see where I'm going on LoS pulls - you wouldn't believe how useful this can be. Plus, paladins are pink anyway.
Also, F1 and F2 beign first and second targets means its much easier, and quicker, to change targets mid fight, which can be essential in fights that consist of waves of enemies.



