If you look in Houchins' book on pages 372 and 373, the distance from the standing breech to the front of the knuckle is 3 inches for the regular frame and 2 3/8 (2.375 in) for the featherweight, irrespective of gauge. I don't have the necessary tools for precise measurements; however, my 20 ga Premier Skeet, featherweight frame, is approximately 2.0 inches from the standing breech to the point where the knuckle begins.
My 20 ga Win 21 Skeet Gun is approximately 2.6 from the standing breech to where the knuckle begins and slightly more than 3.0 inches to the front of the knuckle radius. If you need precise measurements, I can get them for you. Until you asked the question, I never before noticed that the angle between the watertable and the standing breech on the 21 is actually less than 90 degrees, and that makes the measurement a little more challenging.
Also of note, the Premier Skeet's barrels weigh 2.92 lbs and the gun weighs 7.00 lbs total. The 21's barrels weigh 2.41 lbs and the gun weighs 6.27 lbs. I wouldn't suggest the Elsie (at least this one) isn't as tough as the 21. My 20 ga Trap Grade Elsie is far more dainty and much lighter than the Premier Skeet. My prewar 21 has been shot a lot, but it has had a new hinge pin and locking bolt installed. It has also had new single trigger parts installed to correct doubling and the forend loop and short bottom rib repaired - twice. Hope this helps.