Need help with exposure indoors

My wife is graduating nursing school and im going to take a few shots in the autotorium. Im not sure of the lighting thier but its meadium to low light. I dont have my light meter yet so I must adjust acordingly. I'm shooting with a canon 40d with a 70-200mm 2.8 is lens I have a mounted 580ex2 flash but Im not sure that will do anything so far away. What should I shoot at? Should I shoot manual or auto?
I was thinking maybe shooting in av mode

Thanks
Del

Camera: Manual, f/5.6, 1/125”, ISO 200
Flash: ETTL, bounced at 45°-75°.

Adjust f-number and/or shutter speed to change lighting of the background.
Adjust FEC and/or bouncing angle to change lighting of the subject

Del, I think your instincts are good. You will most likely need to stay @ f/2.8 and a fairly high ISO to get usable shutter speeds, especially because you'll be at 200mm to get that happy face.

If you are comfortable with manual, then that's definitely the best way to go. You do not want your shutter speed dropping so low that you miss a shot you really wanted. If you aren't comfortable in manual, then TV will be the next best thing in this scenario. Set your shutter speed to the slowest comfortable setting, probably around 1/100, and shoot away.

Flash is not going to be a good idea in an auditorium if you are sitting far away.

Ups sorry, I missed the part where you said that you’ll be far away..

You guys in this forum are the best!

Thanks

Del