440mm is often considered the minimum and many touring bikes are 450 and longer. But things that affect heal strike are as you noted, the rack, the pannier size and shape, shoe size, where your shoe contacts the pedal and even crank arm length (165mm has a full centimeter advantage vs. 175 mm) and finally your individual pedal stroke-how toe down are you when lifting the pedal or ar you more flat footed. I'm a 44 shoe size (9-9.5) but regularly tour on bikes with 425mm chainstays without heel strike and my cranks are always 175mm.