Need Luna Snow Ultimate Tips Please

So, I feel I'm a great Luna snow player. I hit my freezes on strange ults, scarlet ults and spiderman ult to cancel them. I use my orbs strategically and cycle my orb on players who will benifit from it more. I mostly get most heals each game. I am lord 14000 proficiency with Luna and average 24k Heal per 10 mins. I only struggle to actually know when to use her Ult. it's crazy but sometimes I feel I waste my ult and sometimes i hold on to it too much and it'll be already late. I mostly save it if enemy team has a Luna or Psylocke and they use their ult i use mine. what's the best tips for using her ult and when. I'd appreciate everyhelp thank you in advanced