There is no specific range of values that will indicate cloud cover. Clouds can vary in transparency which allows the underlying reflectance from the land to come through in varying degrees. So there is no set 'lower edge'. You might need to research 'cloud detection in landsat etm' or similar search on the internet. There seems to be several methods that can be used to detect clouds. For example:
http://landsathandbook.gs...cial_Issue_Final.pdf
As for filling gaps - you can do either or both. I would assume if you are going to fill gaps with an SLC-on image that the areas you need to use for filling are cloud-free.