^^ Of course you didn't write that because it's obvious from the top post of this page that it was me. Anyway, it has been corrected and hopefully you are happier than before.
Back to the issue at hand (dorsal width) I calculated the maximal width of the impulse engines' exhaust casing which is 13% (Kimble) or 13.2% (Big Jim Slade) of the saucer width (P/S) and therefore would be 18.4-18.7m wide on an Enterprise
at 305m length or 21.4-21.8m on an Enterprise at 355m length.
According to pictures like this one
the space between the "surfer" skegs has a width of 39% of the impulse engines' exhaust casing, on a 355m Enterprise
the width of the dorsal's stern up there and including the skegs would be 8.5 meters / 27.9 feet
(which is a max. dorsal width figure I had estimated previously).
Where it gets interesting is the ovoid, convex shape of the dorsal in general and the fact that it is "sandwiched" between those skegs
Looks like the stern end of the skegs is the maximal width of the upper dorsal's stern, but there is also the departure screenshot from TMP