Remember also that as with stopping distances in the highway code, perf data is collected by professionals in new aircraft and may bear little or no resemblence to actual performance!
Use the data, apply the factors as per safety sense leaflets etc but think about things in practical terms too.
As an exmple, did a calc yesterday for a PA 28 into our home airfield and apparently it should have run off the end of the runway - though of course it has landed successfully thousands of times without doing so - explain that one!
BTW - it's ruleS of thumb. It is the rules that are plural and not the thumbs!