You need a -6db/decade slope to account for the dipole effect. Take a look at Linkwitz's site. IIRC the xo slope has little to do with the corner of the slope, it has to do with the acoustic properties of your dipole cavity. You should take some measurements of your sub by itself and post them here and we can help you walk through what you need to do.
Do you know how to measure a dipole sub, i.e. extreme nearfield?