If your flying to new eu ops standard then the minimum RVR you can convert to is 800m therefore in the example given above with a 300m visibility, no conversion is allowed.
The difference between vis and RVR comes down to the fact that the runway lighting is taken into account. Essentially vis is what you would see without the bright runway lights. Hence with daylight and no lights, reported met vis = RVR. With HIALS and at night when your staring down the runway the distance that you will actually see is approximately twice that of someone looking down an unlit runway.
Ceiling is only a factor at the planning stage when you are looking at the requirements for your destination and alternates, and then, only if the available approaches are non precision or circling. Once your airborne only the RVR is limiting.