In the file "unit-flags.js" ( ... \steamapps\common\Sid Meier's Civilization VII\Base\modules\base-standard\ui\unit-flags.js ), there is a section that calculates the width of the health bar. It looks like this:
if (this.unitHealthBarInner) {
this.unitHealthBarInner.style.widthPERCENT = utils.clamp(damage, 0, 1) * 100;
}
This calculates the width percentage of the health bar based on the damage the unit has taken.
Now, my best guess is that while the calculated percentage is correct, only 86% of the health bar is actually displayed in-game. Because when you change the number 100 to 86, the health bar now shows the correct value. I could be wrong—but it works.
So, all you have to do is change 100 to 86, like this: