the wall is supporting stuff yes?
then you need to find out how wide it's going to be at the top
so if it's going to be 2 bricks wide at the top, you need to taper it so that it's probably going to be 4 bricks wide at thebottom, the old 1.2 x thingy like the french bloke who worked out this stuff figured out

if it was a 3 foot wall and it was 1 wide wide at the top, the base at the first metre would be 1.2 foot wide, then the next metre down would be 1.4 foot wide and the base of the base would be 1.6 foot wide

then you have to figure the wieght of the wall to determine how much footings you'd need to support the wieght of it

i'm sure someone will tell you exactly what oyu need to know