Hello, i wonder what is the best time to enter world bosses to make sure you start at full hp?

I've tried to enter at once as they spawn, but its often full and way to quick kills.   

I read that its 25ppl at the time and the bosses are up for 2 hours.    Is there a way to see when you enter how much hp the boss has? Or is it better to aim for a kill closer to its disappearance? 

Im only at 5k cp atm, so it would be great to be able to deal some more damage my self before it dies :D

Just press the exit button during the world boss raid if you don't get the boss's hp you desired. Problem Solved.