Ed, thanks for the reply…. if I understand you fully, you are shaving a flat board wider than the shave’s blade?
If that’s the case, then I would not use a spoke-shave, it is a job for a standard plane if you want a flat surface. A spoke-shave is intended for shaping rounded or curved sections, not as a plane substitute. As far as the spoke-shave blade is concerned, I don’t see any advantage in sharpening it to a slight curve as you would with a standard plane – in use it will always be wider than the work that it’s cutting and the corners will never dig in.
To attack small areas on a flat board, you may find a card scraper or Stanley/Record No: 80 more use; these are intended to be slightly curved in their application, producing a very shallow concave cut.
PS…. apologies, I have just re-read your original question and it appears that you are shaping the curved back comb or lower rail on a chair-back, correct?
If this is the case – and this is my method – I’d use the shave to form and curves or chamfers on the narrow edges, but as the flat surfaces, front and back are respectively concave and convex, I would go to a heavily set scraper to form the shape and get the thicker shaves off, refining the set for finer shavings as you get close to the end-shape.
You can use card scrapers, but to avoid bending the thin metal, try clamping two or three together between two pieces of scrap wood and drawing it downwards from each side to meet in the the middle. A bit laborious, but it will do the job.
However, the absolutely ideal tool for this shaping job on chair-back components is not available in the UK, never exported here – though easy to get second-hand if you are in the US – and it is the Stanley No: 82 scraper. Look it up on line. It’s perfect for this job. So, Ed, if you are in the States, get one!