There is no exact torque rating for the preload bolts. However, if there is too much preload, your handlebars will not turn quickly. Also, you will have to play between the headtube and steerer tube if there is not enough preload. So, you can check the load by setting the bike on the ground, grabbing the front brake. Rocking the bike back and forth. If there is any kind of play, you will feel it. You can also see the play in the gap between the bearing cover and the headtube when you rock the bike.