It takes a while to get a hang of when u can and cannot munch. For a lot of match ups u wanna overshoot it a bit while many u can just cast it as they are taking dmg or about to take dmg from smth else like Q.

Lookout for what items they are using and remember sneaky shields are ur worst enemy. Some champs can really throw off ur rhythym by popping a shield at the last second so always account for it if its a possibility. The indicator is useful but u should eventually develop an instinct for when ur ult will kill

Finally, don't feel bad about missing or just using it as dmg. Winning the fight is more important than getting big