Roy, I looked at ALL the pictures of English fowlers in the Grinslade book, and I didn't see any uniformity in either the length or style of the tangs. The only thing noted was the fact that the ones observed tended to have carving around the Tangs, indicating, most likely, IMHO, that these guns were ordered by and built for wealthy English Gentlemen. Considering the social standards of the time, this would be expected of such English Guns. Again, there does not appear to be any mention of the length of the tang for any of the guns in that book. The poster was asking specifically about the length of the tang.
My answer was my attempt to inform him that there was no uniformity, that there is no standard length, or shape to the tang, and that it appears from all the guns I have seen either in the book, or in my drooling over guns at Friendship, both on Commercial Row, and in the Gunmaker's Hall, that the tang relates to other parts of the gun for its length.
My Fowler was made in 2004, by a local gunmaker, who made an American Fowler, at the customer's request, incorporating influences from German, English, French, and early American designs in my gun. I am sure my gun would cause MIke Brooks to have a stroke if he saw it. After all, it has the lock on the wrong side, too!
Since the poster is making his own Fowler, I thought it was appropriate to offer the measurements off my own gun as a basic guide to him.