As I say, find the resultant of the forces that are acting, and if the object is moving in a circle, we call the resultant the centripetal force (which, as you say, has a magnitude of mv^2/r).
The weight, mg, acts downwards. The reaction force, R, acts upwards. So, taking the positive direction to be towards the centre of the circle, their resultant is mg - R. As the object is moving in a circle, this is the centripetal force, which you can now equate to mv^2/r.