Well say for example Dragon Dance Salamence Fire Blasts Offensive Choice Scarf Metagross. That does 88.3% - 104.5%
Its not too hard to get my calculator out and find that means a 28% chance of OHKO without Stealth Rock. But I agree it would be better if the damage calculator handles this - especially as checking with Stealth Rock, with and without damaging weather, for 2HKOs with and without Leftovers, and so on - becomes a fair bit of work.
Also, accuracy probabilities would be nice, even though that is a very simple calculation.
Interface wise, this would probably necessitate adding an extra row in the results boxes, with a box for the number of the nKHO (1, 2, etc), an option for stealth rock damage, an option for whether an orbed opponent attacks between your turns, an option for counting end of final turn weather, and a box for the probability of making the nHKO. Weather and items are handled elsewhere in the interface already.