How Many Pounds?

I figure a 1/4 lb per woman 1/2lb per man and that's ballpark, I'm sure someone else will know better....... I also guesstimate you'll probably lose 2 lbs off the 8lb butt.

I would expect more loss than that. Probably use as high as 50% on a bone-in butt.  You want to error on the side of caution.  Nothing wrong with leftovers, but you don't want hungry people with empty plates staring at you.

# of people X serving size = Cooked Meat, then multiply times 2 to get amount Raw meat needed

For Example: 

25 people X 6 oz serving = 9.375 lbs of cooked meat then multiply times 2 = 18.75 lbs of raw meat

4 oz servings - 12.5 lbs raw meat
6 oz servings - 18.75 lbs raw meat
8 oz servings - 25.00 lbs raw meat
and so on......

It will depends on mix of crowd (men,women,children). Also, if you are doing sandwiches or plates, amount of sides and desserts.  

If it is an informal event and people will serve themselves, you go through more.  Also, if people will be there for a while and get seconds (or thirds) you will go through more.