@homegrown
I'd tend towards expressing that 28.7% is already sufficient to stop recommending mastodon.social, or any other big* server; that's too high a concentration for a decentralised network.
I'd therefore turn around and positively NOT recommend mastodon.social at this point. Obviously, nothing against mastodon.social or it's people, but for the general health of the network, (and for humanity!) Waiting until 40% would be too late.
I would not be so worried to establish at this time any hard percentages for limits or de-federation, I'd just hope that a campaign for conscientious of the reasons why not to join mastodon.social, the importance of small servers, and many of them, would be enough, and then deal with the other scenario if we ever get there, hopefully not.
* I'll admit I have an ideal about #fediverse servers, which is that the "community" of users on a server should reflect a #community that shares a reasonably close connection in some sense, AFK, IRL or whatever way you want to say #offline. This in itself should limit numbers, without having to explicitly place those limits.
Of course I accept other people have other valid notions and do not share this idea, so some larger servers may work too.