So I would set up a porportion: 7 cups flour over 5 cakes. 7/5. 
Then the other half would be: x cups over 1 cake. x/1
Set the two equal to each other 7/5=x/1 
Cross multiply: 5x=7
Divide to solve for x: x=7/5
x=1.4 cups. 
So just used about 1.4 cups of flours per cake.