We take performance data from Runway Analysis Manual which includes runway slope. At some weird runway sloping as at Quito, Ecuador, (El 9200), where you can't see the opposite threshold for example, no performance allowance for headwind is permissible. Likewise, at Rio Negro, Colombia, (El 7000) and at Guatemala (El 5000) the runway slope is such that one can takeoff at significantly higher gross weights going downhill with a tailwind than to go uphill with a headwind.