With the rebate across the back I’d say 4mm is fine. As for the hidden fixings in the inspiration I’m guessing they’ve just stopped the rebate 20mm from the face and cut the shelves down at the front to match.
I’m trying to convince Liz to let me rebuild our Billy’s in the same way.
Definitely do a couple of test joints as mentioned, but assuming accuracy all round, 19mm rebate for an 18mm board will end up feeling sloppy/being weaker across the whole structure
With the rebate across the back I’d say 4mm is fine. As for the hidden fixings in the inspiration I’m guessing they’ve just stopped the rebate 20mm from the face and cut the shelves down at the front to match.
I’m trying to convince Liz to let me rebuild our Billy’s in the same way.