Quote Originally Posted by Unregistered View Post
...yes, it scales on burst spent. And you expend half at a time. So when you cast at 100 burst, you spend 50. So casting it at 20 or 30 as asked means you expend 10 or 15, for presumably +40% or +60% burst damage.
It spends all burst gauge you have, up to maximum of 50.
You can check it by using Roaring Blaze at 0 burst and then PF, it'll eat 20 bg (or 35 with Shingen's spear), not 10.