A guide would be helpful, but I would like to make a few points.
1. I would steer clear of bundling an FTB diagnostics guide with a guide to overclocking. If you want to make two, and link from one to the other, you're welcome to, but I'd advise separation of purpose.
2. Keep in mind who the guide is for, who is the audience, and what the point of it is. From what's been said, the guide will mainly be for regular users who get bored of saying the same thing over and over. The audience is going to be new FTB/MC users (read: relatively non-technical, and impatient at issues encountered). To that end, I'd steer away from technical explanations in the text itself, it'll put off some, and most will simply not be interested. IF technical information is required, it's available, but for accessibility, be careful of the depth you go to.