You can already tamper with the defending Pokemon's HP to see if the attacking Pokemon can KO the opponent at their present HP. The text it will spit out will be something like

*252 Atk Teravolt Kyurem-Black Iron Head vs. +4 0 HP / 0- Def Diancie-Mega: 148-176 (61.4 - 73%) -- 18.8% chance to OHKO.*In the example I posted, I was calculating with Diancie-Mega's HP at 71%. The math is right, but I think it'd be a bit cooler and more streamlined from the calc if instead of

*18.8% chance to OHKO*it became

*18.8% chance to*to reflect the change in HP that the user had entered.

__KO from 71%__