35kts

I guess the point I'm trying to get to is if you have a reasonable expectation of wind speed and water conditions, you may be able to make a better choice of rigs on any given day. Ever been halfway down the run and thought (I should have rigged the so and so?).
If there was a given ratio that was readily available (along with some of the brilliant ideas on this website for fin selection based on sail size) it would make the choice of not only boards but fins for speed sailing on that particular run a lot easier, thereby removing the (I might go with this today, cos it worked yesterday factor).
The idea of getting everything to work together is paramount to having a ball, or having a short uncomfortable blast. (For me anyway) If you speak to someone who regularly does over 100ks in a day, they might say something like "I just felt comfortable so I kept going". So, that's what does it for me, the comfort factor. I'd rather have an enjoyable day with friends on the water, than trying to break my neck chasing numbers. To get all this right, it's important not to have to change rigs mid-session. (Unless you've got a rig bitch) . Life's short, try to enjoy it more..

In very flat water, if you're tuned for it, in light winds I think double wind speed is doable for good sailors.
But this ratio doesn't hold as wind increases!

Speed is dependant on the power to drag ratio, wind speed is only an indirect factor, what is achievable at low wind strength isn't at high wind strengths

There's an efficiency thing happening here. In theory the amount of power available to the windsurfer is dependant on their leverage over the sail, (weight and height)
In lighter winds, bigger gear, stronger winds smaller.

But big gear is heavy and boards and fin have more area, so although you may have the same power in your hands at 15kts as 30kts, there's more drag, so you can't go as fast. But the ratio of board speed to wind speed is high.
And although you have much less drag with small gear in strong winds, and you can go faster, the ratio of board speed to wind speed is lower. So at 15kts twice wind speed is possible, but at 50 kts most people would be lucky to even reach wind speed.
Slowy once told me he worked on 50% faster than the wind, I think he was talking about doing 45kts in 30kts. Which he probably would have done at Liptons the other day if it was a bit flatter.

Just want to add another perspective, that for most people achieving their first 35kt run is not easy, it takes all that has been said here before me. But that's what makes it so rewarding. I remember the first time I rode my mates jetski, I did 100km an hour, which is 54kts at my first try. It felt totally exhilarating and exciting but I had zero sense of achievement, satisfaction or pride in my effort.

Dont know if its a common factor but most of thevreal fast guys here have sailed in the ocean a lot, and a lot of down wind runs. Its not ss fast as flat water but perhaps great training, learning control downwind in ocean swell. All these guys can sail over powered but rig for the gusts on speed runs. The best training is big gear in rough water. When you swap to right size gear on flat water it seems so easy. Guys like slowboat and stroppo might mostly sail on flat water now, but they are using skills they learnt sailing ledge to lancelin or sailing to miles offshore to james service reef. I once heard that Anders bringdal used to sail from mindarie to rotnest island and back for a training run. Flat water should not be so scary after that. Not every day is game day. Train hard so the game day is easy. Works for most sports.