with this ~40' from the deck mast straight up, how much would i have to slacken the forestay and tension the backstay to get ~6-8" rake?
Rake isn't usually expressed in inches, because that value is dependent on rig dimensions. Probably what C&C refers to is the distance between the back of the mast and the mainsheet when the boat is level and the mainsheet is hanging straight down.
But that's not even Step #1 of rig tuning. Before you do that you have to get the mast in column. You can't just go wrenching on the forestay. There's a step-wise process to rig tuning.
Suggest you read this: Adjusting Your Rig
and follow it to the letter.
I'll tell you this: Once you get it right it will have been worth the trouble. I did it season-before-last, improving on it last season, and the helm is just terrific. From air the light side of moderate up to pretty stiff: On many tacks we can just let go of the tiller and it just sits there gently vibrating for a while, and eventually the boat begins to start slowly rounding up
will the stays have to be shortened/lengthened? i dont even know how i would slacken the headstay with the roller furling on it...
Got no clue on that one. Don't have roller furling.