I had cut the sides to the template in the plan, but had added a little material to each side 'just in case'. Marking the finished height of the top of the sides was easy by laying it on a flat surface and marking around level with the head and end blocks
It was pretty much all of the 3mm extra I had allowed.
Marking the finished height of the back was a bit more of a puzzle. I was avoiding making a radius dish, and had read up in the Cumpiano / Natelson book about using a sanding board to shape a radius on the back. Faced with the reality of this (and the need to make a large, flat sanding board anyway), I caved in and made a jig to carve out a rectangular radius dish!
I made it out of 18mm MDF, but after cutting the dish, the board became very flexible and the edges bowed up, so I had to glue it down to another piece of MDF to stabilise it. The whole thing only took a couple of hours, including making the jig.
This is the back sitting on the radius dish before any adjustment - they're actually a reasonable fit. If I'd only added (say) 1mm to the sides, I'm sure they'd sand down to a fair curve quite easily.
As I had added more material than that, the radius dish provided a reference for marking the finished height of the sides at the back of the guitar. Again, this was pretty much all of the 3mm I had added on:
Both edges of the sides were planed back to the marked lines (quite enjoyable work).
And then the back was worked against some sandpaper stuck to the radius dish to level out the cut.
For the front, it was necessary to use a sanding board as the inner parts of the head and end blocks stand proud of the sides:
It was about now that I noticed there was a crease / divot in one of the sides around a patch of funky grain - I made a separate post about this, but the upshot was that I was unable to get rid of it by heating, so ended up fitting a doubler to the side in this area to try and support it, and bring the distortion within 'sandable' limits.
It has helped a bit
Andy