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.

13 ATDC is correct for that procedure.