def clipping_dist(delta):
    """
    Returns the average distance between residues i and j, based on experimental data.

    Parameters
    ----------
    delta : int
        The delta between residue indexes i and j.

    Returns
    -------
    float
        The average distance.
    """
    if delta == 1:
        return 4
    elif delta == 2:
        return 6
    elif delta == 3:
        return 7.5
    elif delta == 4:
        return 8.5
    elif delta == 5:
        return 10
    elif delta == 6:
        return 10.5
    elif delta == 7:
        return 11
    elif delta == 8:
        return 12
    else:
        return 12.5