Rule 1. The answer to this question and the rules you must work to should be in your own outfit's Ops Manual, not on PPRuNe.....
However as BOAC said: "How does cloudbase effect this??
The controlling factor for a precision approach is usually the RVR ( it will be on the plates, typically it may be something like 550 metres RVR at the touchdown end, your allowable midpoint should be your manuals, but lets call it XXX)...cloudbase is usually not limiting and not controlling unless you are dealing with a non-precision approach, but no doubt there are exceptions.
So, generally, if when flying a CAT 1 approach you hear ATC issue RVR's of "550/XXX/XXX" as you reach the approach ban point (which BTW may well not coincide with the FAF for a precision approach), you can continue to DA regardless of the cloudbase (and assuming no equipment failures)
But I must emphasise again - The answer and the rules you must work to in your operation should be in your own outfit's Ops Manual, not on Pprune.....
Last edited by wiggy; 3rd August 2014 at 08:37.