Thanks for that, Rick, I understand now that this top doesn't need to float. For future reference, in what situation would a floating top be required?
The reason i went knock down was I had a job some years ago where we built a lot of desks, and the tops were always screwed on. A permanently fixed top seems alien to me.
Thanks for the pictorial, too. I joined the legs that way, but they were a few mms out. Reading that has taught me where I went wrong.

Hey Chris - if you look at your wood, you'll see that the grain runs along the wood primarily in one direction. The grain is basically made of hollow tubes (like straws) all stuck together along their long edges - the straws are aligned in the direction of the grain. As you add or remove moisture from the straws, they swell or contract across their sides. They don't get much longer or shorter as the moisture changes - they get fatter and skinnier. Because of this, your board will get wider and skinnier as the moisture content changes - but its length won't change much. This change in width (across the grain) will be much more pronounced the wider the board.

In your piece the grain runs along the table top, and along the supports. So when they change in width, they'll do it in the same direction, and by about the same amount. So you don't have to worry too much about moisture changes.

If you'd made a more traditional apron and legs, the grain in the apron would have run perpendicular to the grain in the table top. So if you'd glued it to the table top, and the table top swelled, something would have to give. Your joint, the table top, the apron - something. The solution is usually to allow the top to move by enlarging the screw holes toward the ends of the apron, or having sliding attachments that can move in slots.

Your construction doesn't have that problem, so you can attach the legs to the top more permanently - they'll all swell together.

Hope that helps...

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I don't have as many Festools as Fred. Or Marcou's, or Brese's, or Lie-Nielsen's, or Lee Valley's, or Blue Spruce's, or Harold and Saxon's, or...

Ahhhh, I see. That helps heaps. Thanks, Poto. Here's me thinking the movement was down the length of the board.
You learn something every day.
Sometimes two things.