What’s Your Favourite AM “Rule Of Thumb”?

I’m just in the process of writing a rigorous guide to determining how much to spend on your campaigns (a pretty large task involving Actual Math). However, whilst doing it, I’ve been struck by how many different approaches there are to AM rules of thumb – and how reliable many of them are.

Currently, AM rule of thumb advice is scattered all over the place, so I thought I’d start a single thread to gather it all!

So, come on – what’s your favourite short-line-or-two rule that you use to ru ...