Ratio of sin i to sin R

Would you be able to help clear this up a bit? Regarding refraction of light/snell's law, is sin i always equal to sin R? How is a refractive index determined? Really confused, thanks in advance.

I think you may be mixing up reflection and refraction.
Look at the link; in the link:
'r' means angle of reflection
'R' means angle of refraction
(But sometimes a small 'r' is used for angle of refraction, which can be confusing.)
______________________
Reflection (e.g. from a mirror).
Angle of incidence = angle of reflection
Angle i = Angle r
______________________
Refraction (e.g. light entering glass)
sin(angle of incidence)/sin(angle of refraction) = refractive index (from Snell's Law)
sin(i)/sin(R) = refractive index

For example:
Light enters a glass block with an angle of incidence = 30⁰
It is refracted so the angle of refraction = 19⁰
The refractive index of glass = sin(i)/sin(R)
= sin(30⁰)/sin(19⁰)
= 0.5/0.326
= 1.5 approx.

Yes the angle of incidence (i) is always equal to the angle of reflection (R). If you mean R to be refraction, then that's a different matter.

In Snell's Law

n_1 sin(i) = n_2 sin(r)

where r = angle of refraction. You need to know three quantities to determine the refractive index. Generally in these type of problems the refractive index of air is approximately equal to unity. n = 1. So if you know the angle of incidence and refraction, you can calculate your value of refractive index for your medium