One "formula" I've heard that works well for me is to gradually work up to one third of the distance you'd like to ride on tour. I had a goal of riding 600-mile weeks on my last tour and I gradually increased my cycling up to 200-mile weeks (over many months) and that went well. I got in very good shape and didn't hurt myself and the tour was enjoyable from the first day. I only rode one century while in training, but rode centuries on roughly one third of my tour days.

I suppose you could argue conversely that you shouldn't exceed 300% of your pre-touring distance when you start touring, then cycle yourself into shape, if needed or desired, while on tour (not my favorite thing to do). Of course, much depends on your age, fitness, natural athleticism, etc.