8% to 10% is 25% increase because you're ACC is so low it helps a lot more and impact of ACC diminishes the higher to go. It's the same formula you used, except you used x instead of A2-A1.
If you use H2/(1-H2) for really high haste, like 98% to 99%, you get a ridiculously high number (9900% increase), which makes sense. If you did 0 to 1% haste, increase is ~1%, which also makes sense.
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