I had a test of speed just the other day before seeing this thread.
Here are few examples.
As it was mid day and I had no ND with me, had to use TC to reduce more light for some images (Doubling the F number).
I believe if you can make the propeller blur to cover 2/3 of circle that is for a heli with 3 blades, that makes a 120 degree to cover a full circle, 80 degree of move, you have just a perfect image.
Leeuwarden's first image covering ~ 1/2 or your own image covering 1/3 of split circle are acceptable and nice too but I like Jay's picture as the best that is what exactly I was saying: 2/3 blur coverage. But then the problem is I love whatever Jay shows. And I think the golden magic words are said very correctly by him:
"Better a sharp shot with OK blur than a blurry mess with a full disc."
BTW remember that angle of view also has a lot to say for the outcome of you image.
Look at image 3 with shuttle speed of 1/25. Angle being en face, it still looks prety frozen and not blur.