The first guitar I built, a dreadnought acoustic, I made the neck 22mm thick as the book said, but didn't realise that was with the fingerboard included.
After I built my second cutaway acoustic with a nice thickness neck, I realised the first neck badly needing thinning down. The neck had my homemade single acting truss rod fitted and I wasn't sure how much wood was left in the middle at the back, so I drilled a 1.5mm hole in the back of the neck so I could measure how much wood I could remove.
I clamped the guitar face down on my bench put a rail either side and routed off 5mm as far as I could go before hitting the heel and headstock then shaped the rest with a spokeshave. It ended up with a nice neck instead of the 2x4 size neck before that. I ended up selling it to help finance a new amplifier.