Andy,
You are right about the depth of the 'trough' being '....a little too deep.....', as when I made the original rear bulkhead [of which you are using a resin cast] from styrene sheet, there was a difference from my actual measurements of two Shannons [13-02 and 13-04] and the official RNLI plans of 1.66mm [when converted to 1/12th scale] against the height of the model's wheelhouse. I decided that the extra depth would impact on less [all the other splendid working parts which you have made and more] than by making this correct and everything else having to be altered too. However, at this stage, anyone can add a thin styrene template of 1.55mm [plus paint = 1.66mm!!] to the shape of the gully and..........hey-ho, you should be back to normal [or so].
All in all worth pointing out, which at the time I did request was made on a small 'errata' sheet for modellers getting their models.
Cheers,
Kim [C.C.]