Cast bar is not accurate

[regel]

When I cast Incinerate an a dummy target I am able to cast next incinerate when 60 - 70 % of the cast bar (yellow) is used. The blue bar is just a little bit further.
My latenzy is around 70 ms and I have approximently 65 FPS.
Can I change the castbar so it is more accurate? I tried the standard WoW castbar and it had the same "problem".