On fly sizes 16 and smaller, Gary Borger factors in the length of the hook eye as part of the insect length. Gary specifically discusses the hook eye in Designing Trout Flies pp. 178 :
"?.tiers have not considered the eye to be part of the representation of the food organism; rather, the eye (like the hook bend and point) is regarded as a necessary evil.
Thus when selecting hooks most tiers consider only the length of the shank (from the rear of the eye to the bend). But in fact, the eye should be considered in the overall length of the imitation especially when tying flies sizes 16 and smaller. In these smaller sizes, variations in length are more evident than in larger sizes (page 46). Time and time again, I've had highly selective fish refuse my fly only to readily take the same imitation tied one size smaller. In these situations, the length of the hook shank of the first fly I chose was the same length as the natural's body. For the fly one size smaller, the length of the shank plus the length of the eye matched the body length of the natural and thats the fly they wanted. Thus for flies size 16 and smaller, the eye must be considered an integral part of the overall length of the design.
Some designs lend themselves very nicely to hiding the eye?? In other designs the eye sticks out like a proverbial sore thumb. In these instances, consider the eye of the hook to represent the head of the insect"
I've heard other authors say many times that if you are certain about a hatch, but the trout are refusing the fly, go down a hook size. I think the eye of the hook lengthening the "apparent" body length to the fish may be a reason.
More food for thought when considering the hook eye.
"Discovery consists of seeing what everybody has seen and thinking what nobody has thought"..........Szent-Gyorgy