280 should be pretty damn close i would think. I cant imagine you need more then a 290. You can only rough it in by eye. get a sync tool and you can sync your idle and your slides. Change idle by each of the screws on the side, Or if you have TORS adjust it on top of that dumbass looking doo hicky on top. Slides you can get pretty close by eye and sound of the slides but idle your going more by sound and feel, and maybe your pipe temps and smoke output. Get a sync tool it takes the guesswork out. Only like 40 bucks from a site sponsor like FAST or even on amazon.