For Ambient temperature increase to 50°C as a rule of thumb, derate the motor capacity 10%.
Exact calculation is complex since part of the motor losses are constant (core loss, friction and windage) and the remaining losses are proportional to the motor current squared (stator I^2*R, rotor I^2*R and Stray load loss). The reduction of load will attempt to reduce the motor current and then the temperature rise by 10°C, this will allow the extra 10°C on ambient.