Don't have time to do a precise study of this at the moment,
but I just did a quick ratio check using photos where the Renton
gull and other GBBG are in reasonably straight profile and
relaxed posture (with neck not stretched) measuring -

- distance from the point where the cutting edge of the bill meets
the facial feathers to the back of the head on a straight line
extending from, and parallel to the cutting edge
- distance of a vertical line from the lowest point of the belly
(usually directly between the legs) straight up to where it
meets the back

On the Renton gull this ratio is consistently a bit less than 1/2.
On all 15 (random) GBBG web photos I tried this on the ratio
was always at least slightly greater than 1/2, and on avg it
was closer to 3/5.

This was quick and imprecise, and the sample is too small to
mean much of course, but I will study this more carefully when
I have time. Hopefully it will help illustrate in hard numbers
how round-bodied and small-headed the Renton gull clearly
is to me compared to the vast majority of GBBG. Just wanted
to throw it out now in case anyone else wants to try it.

Cheers,

Phil