Calculating Ingredients When Feeding Large Groups

I use a book called,"Food for 50". It covers serving sit down meals, bufetts, and many other topics we often don't consider when calculating how much food to prepare.

You should be able to find it using the ISBN number below.
ISBN 0-471-27030-X

Right off the top, 10 pounds of ground beef made into BBQ (or sloppies) will feed 20-30 people, depending on bun size. I use the party bun size for a sandwich accompanied with other meal items. Which with your list, I would use it as a side, not the initial component. So really, 25 lbs ground beef will give you a little wiggle room. Making the potatoes from scratch, I would do 30 lbs of potatoes. 10 lbs prepared cheese potatoes makes a 20 lb turkey roaster full.
We do large family gathering all the time. I cook at church during lent, make soup. I am doing turkey noodle (roll out noodles) for 200 for Ash Wednesday. Using 1 20 lb turkey for that. Will have plenty.

Go to this website and explore it: lotsofinfo.tripod.com/index.html I used this website a lot when planning my daughter's wedding on a shoestring. They will have diffent pages for planning for different numbers of people-how much you need, lots of different recipes! Very helpful! Over 500 Make ahead or Large Recipes and many Self Catering Tips --- enjoy! Here is the address for this page, if you are 'trapped' in frames:
bigrecipes.com

Allrecipes will work well "but only if you're going to be using one of their recipes". If you're going to be using your own recipes please just do what Suntdy recommened. You'll also want to factor in that not everyone is going to eat all three items on your menu plan so plan on factoring cooking each recipe for less people than you're expecting.

You'll also need to take into consideration if you're serving appetizers and salad as well. If you want to play it safe you can always cook it all up for everyone you're expecting and freeze what's left over. ;-)

If you go to allrecipes.com, it allows you to calculate recipes.

Look at how many servings your recipe would normally make and calculate. For example, say your sloppy joe recipe is for 8 to 10 people normally. You are looking at feeding 60 to 70. Say your average for the ingrediate is 9, 9 x 7 is 63 - 9 x 8 is 72. Go with the lower number and multiply all your ingredients by 7. You will end up with a number of servings between 56 and 70 (according to the original recipe multiplied by 7).

Google it, and also some cookbooks have lists like that. Look in some of your cookbooks and see if you can find anything on the topic.