Evo, Airbedane
You're correct -- I leapt in quickly without sight of the specific question. Interpolation in the way that you suggest may be the best that one can do without further info -- though it's still very unreliable. You can easily find significant counter-examples on a 214. Looking at isobars helps a great deal.
But I stick to my guns about the ludicrous approach taken in teaching and examining this area. I remember a question on a CAA met exam asking for the average wind over a route for which the only option available was an arithmetic mean of speed and direction. It was clearly inappropriate.