R is the radius of the circle, and it can be any number which is greater than zero.
The ratio of L1/R is the same (mean the same angle).
For example: if you only have index of refraction for both media, and you do not have a calculator to calculate the value of sine function.
You can draw a circle with any radius R. With any incident ray, draw the horizontal displacement L1.
Then you know how to find the outgoing ray: the one with horizontal displacement L2=n1*L1/n2.
And I believe if you ask students to do this kind of exercise by hand, they will remember the refraction law.