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IRL it would be less than 31mm (it's rim dependent but the tyre nominal width does not equal tyre height).
Plus, as Ed said, a lot of OTP bikes have a way too high BB to start with. I much preferred the handling on my Straggler with 650B wheels, although it was designed for the same sort of volume on 700c.
Lolo-
It's a very near estimate and if anything like @MisterMikkel said tyre sag will probably take it to that 31 mm even if the tyre isn't as high as it is wide. OTP bikes' bottom bracket are high so you don't get pedal strike while cornering or while you're out in the woods with squishy capable tyres. Moreover lowering the BB by altering the geometry would slack the frame and alter the ride even more for you'll have to lengthen chainstays & DT. You may personally like how your straggler rides with smaller wheels (I call placebo but I'd be interested to try). All this makes me think going from 622-40 to 584-47 definitely makes less sense than putting 584-38 or 42 mm on a bike that wouldn't take anything larger than 622-28s, for you more or less keep the same ride. ✌️
Heldring
Re @edscoble and @MisterMikkel sure you are right about volume and ride quality but going from 40-622 to 47-584 will equal a bb drop of 31 mm which is less than desirable depending on what's the 'optimal' (ie the tyre size the producer had in mind) size for the frame