Use worst case scenarios, so at 40 degress, 2000 ft pressure height with a 10kt downwind the aeroplane will take off and land in XXXm feet etc. If the conditions are better than that, you are covered. I have never met an ATO that would tell you this was wrong.
For my CPL i used this for my Navs and test. I would only get the p charts out if i was on a very short srip and wanted to cover my behind on the actual conditions of the day.