The lowermost ionosphere (the D-region, below 90 km altitude) is the most difficult region of the atmosphere to probe. Too high for balloons, too low for satellites, it can only be probed either by occasional rocket launches or by radio waves (either HF or VLF). In particular, the propagation of VLF signals has been shown to be a sensitive indicator of the increase in electron density due to enhanced X-ray ionization associated with solar flares. The increased electron density causes the VLF wave to reflect from a lower altitude and VLF observations are often categorized by changes in this reflection height. However, quantitative modeling has been hindered by uncertainties in the details of the solar spectrum. The only operational system that measures the X-rays relevant to the D-region are the NOAA GOES satellites which only have two broadband channels. We therefore use two models of the solar X-ray spectrum, one, an empirical model and the other, a coronal loop model, as input to a physics-based D-region ionosphere model to quantify these details. We also look at the time variation of the ionospheric flare response and attempt to link it to specific wavelength bands in the solar spectrum.