PLEASE HELP ME! SOS... MayDay... Pleaseeeee

I NEED HELP - My Boss just dumps all the beverage reponsilbilities on to me - and the EVENT IS 3 DAYS AWAY!!! Please help me!!

1) How do I estimate the amount of alcohol (beer, wine & liquor) we are going to need for a 3 hours Cinco De Mayo Event for 800 Latin people? We are having:
* Three (3) beverage concessions
* Live music (2 bands)
* Buffet Style Food

2) Is their a basic formula, ratio or a gauge to go by, on how much beer, vodka, whiskey, mixers, etc to purchase?

3) When you purchase liquor/beer from a local beverage supply company. Can you return (as a general rule) any unused/unopened products?

THANK YOU SO VERY MUCH FOR ANY INFO THAT WILL BE BENEFICIAL TO ME!!!