I try to keep the tank hovering above 80%, generally. This way I don't need to worry about it.

For raids, if I'm familiar enough with a fight to recognize tank busters by name then I'll let them drop down to 30%. I, also, like to time my aoe heals to hit immediately after a boss aoe.

For dungeon mobs, I like to play limbo with their health bar. How low can you go? However, I don't want to make the tank nervous so large pulls don't typically drop below 40% for anyone. Bosses though I'll make sure they stay alive.