So...after checking with the "string method" (actually using a wire cable) I found that the left measurement was 2mm shorter than the right measurement. That means there was some small amount of left thrust.

To correct this I had to completely removed the engine, cut down some of the wood around the engine and enlarge (and file) the holes in the mounting platform. Now I've managed to get that 2mm shorter distance over on the right side (so a slight amount of right thrust).

The method that the model uses to mount the engine (beams with ply "shelves") is not new. And I think it's intended to make straight engine mounting fool-proof. Well, I certainly proved THAT wrong!