If someone has teeny weeny lashes I always put it the whole lash as I don’t have time to redo them, they need all the help they can get.
Generally speaking a good 2/3 of the lash. Any less and I find a real limp lift is the result, particularly when lashes are very strong (if they take ages just to glue down and ping back up).
If client has hooded eyes, like myself, if you use a smaller shield and lashes are a good medium length or long, you will not want to put too high up as the lashes will look great for 5mins then get eaten by our creases which are low down, then end up all over the place, hooked ends and totally curled back on themselves over time. However, using a large shield and applying over whole hairs works very well as i’ve found out on myself. If I only put on the root or some of the hairs it’s a total waste of time for me sitting there for an hour doing them on myself.
This isn’t stuff I learned during training it’s just what you pick up as you go along and what works for you, so don’t be afraid to try things.