Old Smokey,
We use manufacturers data that is based on the ISA, so it’s a standard lapse rate. Nothing additional is built in for Temperature Inversions.
Our policy is to account for ALL of the reported the inversion starting with fixed derate plus assumed, increasing to maximum thrust and different flap setting if required. Failing that, remove payload.
Nighttime inversions of 20°C are common during the summer months. We started accounting for the complete inversion following a crew report that they had a -50ft DESCENT rate after takeoff with all 4 engines running
Mutt