Making a Grade Book

I am currently trying to make a grade book. I noted a flaw in the original design and am trying to perfect it. Sadly I am having no success.

I've attached my file, so feel free to take a look at it.

So here's what I've done.
I have a column of names
Next to that I have a row of Assignment names.
Below that I have what the assignment is out of (any number not percentages).
Below that I have a row of weights for each assignment.
In the appropriate columns, the marks will be enter for each assignment.

What I want to be able to do is simply have excel process their mark by considering the following
a) calculate the percentage for each assignment
b) take the percentages and apply the weighting system I decide upon.
c) provide a final grade in a percentage

The problem I have is that I don't have all of the cells in the spreadsheet utilized, but want to have extras in case i need to add an assignment. With that being said, the formula I begun to write states that I have a Div/0! error. I understand what this error means, but am unable to fix it.

I would love for anyone to look at what I've done and give me a hand.
Here's my formula:
=((D28/D$2)*D$3)+((E28/E$2)*E$3)+((F28/F$2)*F$3)+((G28/G$2)*G$3)+((H28/H$2)*H$3)+((I28/I$2)*I$3)+((J28/J$2)*J$3)+((K28/K$2)*K$3)+((L28/L$2)*L$3)+((M28/M$2)*M$3)+((N28/N$2)*N$3)+((O28/O$2)*O$3)+((P28/P$2)*P$3)

I'm sure it's even more messed up now than originally, since I've tried so many different things.

Thanks in advance for your help,

Greg