Right now it is ambiguous as to when you have reached the point of no return and should absolutely stop casting unless you have your ult ready. Good way to clarify this both for new players and for experienced players who are in the thick of combat is that the percentage number turns purple when at 97.5% or above. This is a QOL improvement that goes along well with a feature already added where the number very briefly turns green when you quell below the point of no return, allowing you to know when you can cast something again without blowing up.