Proper massing and buttressing ensures that an arch will stand the test of time. For now let's look at a way to effectively calculate the minimum amount of buttress that an arch needs on its sides. 

1. Label the two end points of the arch's spring line "A" and "B".

2. Divide the arch into three equal spaces by placing points "C" and "D" evenly apart.

3. Draw a line through points "B" and "D" (feel free to use "A" and "C", nothing magical about which pair you pick)

4. Take note of the distance between points "B" and "D"

5. Extend the line between points "B" and "D" downward and in the same trajectory and the same distance to create point "E". 

6. Point "E" defines the minimum amount of buttressing that the arch needs. But go ahead and give it an extra 4" of meat, better yet make it an even 8".