I have had a lot of experience with this and from my point of view the first thing to create is a way of tracking sales.
This becomes your "par" if you will.
Keep track of the items that move better then the others.
Count customers through the door.
Keep track of "rushes" and what time of the day they USUALLY happen.
Starting out from this stage will be by trial and error at first.
After the second or third week, you should have a pretty good idea of what sells and what does not.
Knowing this information you can now create prep list.
These lists can be daily or weekly or anything in between, but they must contain specifics,
if you are going to use this information to keep tight inventory controls.
Here's an example:
2 gallon Italian bread
1 gallon raisin cinnamon bread
1/2 recipe eclair dough
1 quart pastry cream
This way the prep staff knows what they have to make.
For banquets....I always made lists in gallons of quantity. If I had a group of 100 ppl and the serving soup size was 6 ounces, I simply multiplied 6 times 100 which equals 600 and I divide that by 128 ( the number of ounces in a gallon) now I know that I need approx.21 1/2 gallons of soup plus a little extra, just in case....