The only question I have with regards to this design question was the selection of R. In the solution provided, they chose to determine the value of IL based on Vo and then choose an approriate value for I such that Iz > Izk. As you can see from my work I did none such thing and just simply let I be 12mA. Is there any problems with that? As a result my R value came out different but that's okay right? Edit: Note in the figure there is only a portion of my solution.