Projectile Peak Height versus Initial Vertical Velocity

Set the target height and the tolerance above and below this height (epsilon) that is allowed. This determines a nominal initial velocity for achieving this height where the red vertical line meets the horizontal axis; and an interval of initial velocities that produce heights within the allowed range. The spread of this interval is indicated on either side of the red line.

The graph gives you an interval in the input (velocity) axis, with the nominal velocity marked. What is the largest interval of velocities that is symmetric around the nominal velocity and produces heights in the allowed range? How did you find it? Will smaller symmetric intervals work?