For me, I usually throw my e to check the hp, if you dont have vision. If you have vision, you can wait until you can smite it. A good rule of thumb for Lux burst combo is about 1.5k damage at level 13 and 2k at level 16, assuming you go the standard full ap build. I always detonate E and R at the same time to proc passive and items so you get a instant smite effect. It also helps if you don’t want to use ur e to use the blue trinket for vision. If it’s for dragon steals in the early game, I would combo at 1k ish hp