I took a class from Kaye England and of course she sells her own rulers for triangles...
She told us you can either do the math or buy the tool and either way it will work. As for Kaye the rulers don't have to be calculated and if you have to drag out a book or check the internet every time you need to do those triangles having a ruler sure makes quick work of the job! LOL
As for me, I do both depending if I am at home with my rulers or in a group situation like a class