--I don't know how to do exactly what you want...
1. You can flatten all nodes if they have lots of different values by dragging them against the floor or ceiling of the track, letting go, then dragging them back where you want them.
2. You can enter a precise value for a single node and then use snap to snap the other values to the same point.
3. You can delete all nodes after a certain point and then move the first and last nodes up or down. Or you can enter the precise value for both nodes.
I have never needed to set a range of nodes to precisely the same value.
I have before wished I could move a set of nodes up and down relatively adjusting rather than with a fixed difference between all the different nodes.