With a gasser (and hopefully a glow) you should do 2 radio checks. One with the engine off, and the other with the engine running. If the two checks are different by more than 10 or 15%, then don't fly until you isolate and fix a problem that was indicated by the range checks. Personally, I use a minimum distance of 125 feet as the least amount of range I will accept prior to a gltich. This is in addition to the 10-15% rule.