How to compute thrust for SET_ATTITUDE_TARGET messge?

Hi, first of all thanks for awesome software, I want to use GUIDED_NOGPS mode to navigate my drone to desired location, I have compute distance between drone and the destination, and have copmuted roll pitch and yaw angles as quaternions, but cant compute thrust to set in set_attitude_target_encode message with help of dronekit message factory, how can I compute thrust for every update message to handle altitude smoothly.