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After years of looking for a more mathematical way to do it, I've settled for paper template (2 pieces of paper smaller than the alcove overlapping, mark the odd shapes by folding or whatever method you prefer), then hardboard/any thin board from that template, followed by cutting the shelf and final planing (as opposed to sanding). Longwinded but it saves a lot of f'ing and blinding with shelves that always need a bit more taken off, until they need a bit put back...
Floating shelves to be fitted into an alcove. Presumably it's just a case of trial and error to sand mil by mil off the edges to get them to fit snugly? I don't want to chisel into our new plaster and embed the edges there - instead I'm planning on just having 4 good screws into the wall. euph.