No percent or virtual units! Only real kilograms, meters, seconds ... Only real physics!
By adjusting the thrust of the engine, land the lunar module on the moon, the speed at the contact with the surface should not exceed 5 m / s (soft landing). The engine mode is determined by the amount of fuel and the time for which this fuel is fired. If the acceleration exceeds the allowable value, the control is switched off for a time proportional to the acceleration.
Also added a simpler game mode with continuous control, where you need to adjust only the fuel consumption to the engine.