There are apps that can work this out for you

So either i say im going to sleep “now” and it tells me all the times that will align with light sleep and i can choose one to set an alarm for. (It even accounts for the average time it takes people to fall asleep)

Or i can say i want to be up at X time, and it tells me the best times to put my head on the pillow (phone off and eyes closed actually trying to sleep)

Sometimes it works out better for me to get slightly less or more sleep if it means waking during light sleep.