I would order it long enough to contain the whole luff. ATN is quite adamant that it be several feet short of this, because it makes the sock easier to raise. However Bob Johnstone (J-boat) says get it full luff length. The issue is that a few feet of both luff and leach have to be out of the bell before you can start raising the sock, otherwise it will catch and the corners and foot and bunch up the sail. This is true, but if the sock is several feet short of the tack and clew, you have kind of a mess when you get it out of the bag, and it is messier to leave hoisted and socked. Johnstone says get it long enough to sock the whole thing, then remember to pull a few feet of sail out of the bell before you hoist by hand to prevent the bunch up.

I have an ATN for both the asym and the mizzen staysail, both are short by the amount recommended by ATN, and I wish both were long enough to sock the whole sail.

By the way the Rollgen does not work like a rope luff code zero. It is a separate line tied to the spinnaker only at the head and tack. When you roll it up it rolls the wail around this line starting at the head, but when the sail is flying it flies clear of the line.