I think it really depends on the child what is restorative. My youngest DD doesn't nap anymore, but we had all variations of nap length up to that point from 1.5 hrs, 45 min, 30 min etc. Also some kids just are not happy being woken no matter the nap length! For you I would suggest maybe pulling the nap ahead a bit (say 12:30) and capping maybe 15 min to start and see how that goes. Also then bedtime could be earlier as well.