How do you fill out the Takeoff Performance chart if pressure altitude is below sea level?

All the aircraft for which I contributed to the preparation of the AFM had a caption that stated to use the sea level curve if pressure altitude was below sea level.

Yes, that means you take a cut in performance compared with what the plane could actually achieve. And that is on top of the AFM charts being already the result of a *conservative* statistical collapse, as far as the weight, wind and runway slope are concerned (and the wind is ALSO factored: it counts for 1.5 times the actual effect when performance is degraded, and only 0.5 when wind speed and direction improves performance; that is why the tailwind and headwind correction curves have such different slopes).

You don't double guess certified performance.