This is from Wisconsin wildlife. As you can see, they say girth is to be measured without the fins. I've yet to try this for all fish, but I will. And I will weigh the fish I keep to make sure it's accurate. I have a food scale at home, and I always keep my fish on ice until I get them to where I can weigh them.
I've started a list of formulas for estimating weights for various fish and have several different ones so far. I may actually start measuring and weighing some I catch just to try out the formulas.
As for girth measuement, I think I'll try to measure with a cloth tape from the base of the dorsal fin on one side down around the belly to the base of dorsal on the other side. I might also try same fish with fins down and fins up and then try some formulas to see how much difference it makes.
All of this is assuming I finish my honeydews and ever go fishing again.
Well. Yesterday, I caught a nice 7 incher. I know because I measured it.
I estimated its weight using two formulas, and they came out to .34 lb(5.44 oz) and .28 lb (4.48 oz). Actual weight? 3.8 oz. Dressed weight, 2 oz. That's a yield of 52 %. So the weight formulas are an estimate at best.