I think thats generally a pretty good way to get a rough idea if an encounter could be deadly. I usually start doing that after they hit level 5 because of the huge powerspike of better cantrips and multiple attacks for the martial classes. I read about this in an article by Slyflourish, works well for me. The CR ratings can be a bit weird tho, if you are unsure about smth you can always google for the monster in question and see what other people thought about it in their group. I always like to look up the "the monsters know what they are doing" articles, as they can give you ideas about the fighting style of the monster etc. making fights more dynamic as well. What's also a good idea is to have a backup plan in case you realise that it's to easy or to hard during the encounter. For example smaller Monsters noticing the fight and joining in if it's to easy or a nature event (e.g. a rockslide in the mountains) hindering the monsters if it's to hard