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.
EGPFlyer you are absolutely correct. That was probably not the best example on my behalf. I should also add:
Converted met vis must not also be used for take-off.
Converted met vis must only be used when no RVR is available.