lab-5-6-brightness
Lab 5/6 Error Bound Finder -- Brightness context
If you give this applet a certain Δd, it will automagically calculate the resulting error bound (while simultaneously drawing the picture to show you what's going on). You can use this to rapidly check whether your approximations are accurate enough for Lab 6.
You will, of course, have to show all your work for finding the overestimate and underestimate in your Lab 6 writeup; this tool just helps you figure out how small Δd has to be first.