Mass-editing formulas?

Hello,

I am trying to create a gradebook for about 300 students. Is there a way to batch-edit functions, and also to allow flexibility in formulas for a varying number of columns?

What I want to do is this: I have the students' names in A. Their grades go horizontally in columns B through Last column - 1. In the last column, I want to show the students' percentages. Each column represents one assignment, and since I don't yet know the total number of graded assignments, I want this to be flexible without requiring the rewriting of every formula.

Is there a way to, for example, click on a column (the "final" column of my gradebook) and insert a formula that basically says, "for each row, take every value in cells from column B through the column before this, add together, then divide by the sum of the "points possible" row (row 1)"?

Clear as mud? Since all I want to do is have each row add all its numerical values together, however many there are, and then divide by the sum of all values in the "total points possible" row, I don't want to have to encode the same formula, varying only for which row I'm in, 300 times!

Thanks everyone,
Michael