No formula, you need do video recording of your skill cast in training room with 60 FPS. Then use video editing software to slowly play back the attack animation frame by frame. Count how many attack animation frames you have at current speed. You start with low value like 50% attack speed, then increase the speed slowly until you find that the number of the animation frames has been reduced. The % of attack speed when your attack animation has reduced frames is the break point. Continue do this till you reach attack 200%.
The reason behind this is the game simulate you attack faster by reduce the number of frames in your attack animation. As D4 game engine locked at 60FPS, the game calculate the % attack speed matching minimum one frame then only it start to minus one frame. The game engine cant reduce half or a quarter of frame, that’s why there is % speed break point which mark the moment animation frame is reduced.