You don't drop from a set of five to a set of three, then doubles, then one single. You keep the general amount of total reps, so 2x3, 3x2, 5x1.
Further, you don't adjust the weights at all until you've been through a round of running it out. Intensity day always progresses at the same interval generally regardless of whether you're doing a set of five, triples, doubles, or singles.
So, for your next deadlift workout, you'd do two triples at 180kg. The next week the same at 182.5kg. When you miss, move to three sets of doubles; keep the weight increment increase the same. Once you've worked your way down to singles and failed on those, then you start over again.
This is detailed in section 2 of the Texas Method chapter in PPST3.