May the vehicle ramp be anywhere behind the starting line, or must it be centered behind the starting line (i.e., can a ramp be positioned as far to the side as possible, as long as it is still behind the starting line)?

Rule 5.d. only states that the Ramp and Vehicle must be entirely behind the Start Line. It does not specify where. So it could be way behind or far to the left or right. Competitors should realize, though, that the dowel in 6.d. must be at least 0.5 m on either side. Event Supervisors could make that distance as long as they want as well.