While the concept of non-simultaneity of events in different reference frames is a well-established concept in physics, using it to justify the possibility of prognosticatory dreams is a stretch. The maximum temporal distance between events that are simultaneous in another frame is not a fixed value that can be used to predict the future. It depends on various factors such as the relative velocity between the two frames, the spatial distance between the events, and the observer's perspective.
Moreover, the idea of using non-simultaneity to predict the future assumes that time is a linear and predictable concept, which is not necessarily true. The concept of time is still a subject of debate and there is no concrete evidence to support the idea of being able to see into the future using this technique.
In addition, the speed at which an observer would need to move to achieve this supposed ability is also not a straightforward answer. It would depend on the specific scenario and the relative velocities involved.
In conclusion, while the concept of non-simultaneity in different reference frames is an interesting concept in physics, it cannot be used to justify the possibility of prognosticatory dreams. The maximum temporal distance between events that are simultaneous in another frame is not a reliable measure for predicting the future and the speed at which one would need to move to achieve this ability is not a simple answer. It is important to approach such claims with a critical and scientific mindset.