Calculating Required Voltage Drop In A Circuit

Here are some sim results for a typical LED, showing Vf and apparent resistance plotted vs. current. As you can see, Vf climbs as If increases, but the variation is in a relatively small range. The apparent resistance, on the other hand, covers a huge range, spanning from over 5k down to below 43 ohms, just in the 0-50mA range I simmed.