belly or top radius?
It looks to me as though by measuring across the top you are measuring top radius. Many guitars are built with a radius in the top and back. I build mine with a 15 foot radius on the top which gives about 4mm clear at each edge measured the way you are doing it.
I would measure belly by putting a straight edge from the end of the fretboard to the bottom of the body, obviously alongside the bridge. Then you can see any dip towards the soundhole and belly behind the bridge.
Slight belly would be normal, due to string tension etc.
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