If a light ray is traveling from an object(clear plastic) into the air, and the refractive index for air is 1.00, how can the refractive index for clear plastic be calculated

You can use Snell's Law that relates the angles of the ray in the clear plastic and in the air to the ratio of their refraction indexes (n):

If you measure the two angles by experiment, then you can calculate the index of refraction of the clear plastic:

and 1and 1