I don't know if there is a correct length.I started using Gold Tip carbons last Spring and footed every one I have made.I felt it shouldn't ride up on the shelf also,so have been cutting all of them 1".I see people using longer ones so I suspect it probably doesn't hurt if it does come up on the shelf.
I haven't hit any rocks with mine but have shot them a lot and made lots of solid hits on wood and made one dead center hit on a steel post.It was so dead center,it just dropped to the ground in front of the post.
I have yet to break an arrow.My arrows are over 700 grs so they hit hard too.I did have an antelope shear a shaft off.He was hit through the shoulders.I also put a 1/2" footing in front of the nock.
I never did shoot the carbons much without footings so I'm not a good one to make comparisons but I have to believe they help a lot.These are the toughest arrows I have used in over 40 years.
I used fiberglass arrows a lot in the late 60's and early-mid 70's.These were tough shafts but if you made a frontal hit on a tree or rock,it would drive the insert back in and split the end of the shaft.In the early or mid 70's,someone came up with a very small,aluminum collar that wrapped around the end of the shaft and had a lip that wrapped over the rim of the insert and the back of your screw in head,seated on this lip.That little collar transformed those fiberglass arrows making them almost indestructible.
I chuck my footings into a small drill press and use a small mill file to bevel the rear edge more than 45 degrees.Kind of like a lathe.It takes a little time but well worth it.