It is an average at each stage, discluding ingredient types that don't have grades (such as basic ingredients).

So for the potion of evolution, it will be (53 + 100 + 100) / 3 which is ~84.

Then that gets mathed into the evobar; If all the ingredients you used for the base evobar are 100, and your giant clover oil is grade 100, then the final evobar will be (84 + 100 + 100 / 3) which is ~95.

So yes, this should be plenty high enough of a grade for evolving a non-legendary.