The overall length (o.a.l.) listed on the drawing is the SAAMI (Small Arms Ammunition Manufacturers Institute) specification. Most of the load data in Modern Reloading was supplied to Lee by the powder manufactures.
One can exceed the recommended maximum o.a.l. as long as it doesn’t interfere with the function of the rifle. If your rifle will not accept ammunition loaded to a longer o.a.l. choose a different bullet/powder combination.