Night shooting advice needed.

Hello. I have a Sony alpha 6400 (aps-c). Need to shoot at night in outside football stadium. Graduation event... not fast moving. Using Sony 600mm lens (900mm equivalent on aps-c). I figure to set iso to auto, stop it to 6.3, then .... what exposure would be ideal? Thanks in advance!

EDIT: Sony 200mm to 600mm lens 5.6-6.3 shooting from monopod in a bleacher seat. Starts at dusk and ending in the dark.