This paper describes a one-directional iterative technique which converts from slant Total Electron Content (TEC) to vertical TEC using information describing the current state of the ionosphere. The method combines the well-known Chapman function of electron density with a spherical harmonics representation of the peak density over a spherical surface. Several parameters either have restricted movement or are kept constant in order for the problem to remain manageable. This technique is compared to the standard thin shell model obliquity factor in order to assess the degree of improvement in accuracy and the conditions under which this occurs. The Parameterized Ionospheric Model (PIM) is used as a truth model.