In uncoordinated settings it can just be a 50/50 if you cant get your whole team to time some burst. A basic tip is to always save your burst damage until right as you smite.
That being said, start by looking at the damage of your smite. Depending on your reflexes, decide a health value slightly greater than your smite damage at which to smite the objective.
For example, if your smite does 800 you can decide to wait for 900 before smiting. Your reflex time might be such that in the time it takes for you to recognize the health threshold has been met and for you to push the button, the objective will be in smite range.
Naturally this has the risk that you might be faster than your team's damage, but smite timings really come down reflexes and damage instinct.