Alright I think I got it. I rotated the crank until the slot was visible in the AB light hole. I then rotated the crank 20 degrees counterclockwise and slid out the IP until it disengaged. I then rotated the crank around to 5 degrees btdc and slid the IP back on. When I rotated the crank to 15 deg ATDC, the slot was right in the center of the AB light hole.

Now I wanted to set my timing at 26 deg instead of 24, so I rotated the crank around to 13 deg atdc and then rotated the IP until the slot was dead center in the AB light hole.