Hi Simon,
I haven't worked out the height but I imagine you have this right given your research and drawing skills.
In regard to the shorter main drainage area on the starboard side, this appears to be in place on all VIICs, VIIC/41s and VIIDs regardless of when they were built. There must be a reason for this. I presume it is because there is something on the starboard side just under the deck which is not in place on the port side.
The main drainage area on VIIBs was the same on both sides so it is something which is specific to VIICs, VIIC/41s and VIIDs.
If you need more info on this please let me know. The way to check it out is to use the main drainage holes as a guide.
Cheers,
Dougie