I recently discussed with the creator who informed me that the modifier associated with rolls for lingering injuries is calculated as the PC's Constitution score minus the damage from the attack. So, if a PC has a base Constitution score of 15 and takes 20 damage from an attack that reduces them to 0 HP then they would have a -5 modifier applied (15 Con - 20 dmg = -5 modifier) to the lingering injury roll. Conversely, if the same PC with base Constitution score of 15 takes 10 damage and is reduced to 0 HP then they would receive a +5 modifier (15 Con - 10 dmg = +5 modifier) to the lingering injury roll.

I believe the idea is that Constitution is a measure of vigor, while a single powerful attack of high damage increases the likelihood of suffering a devastating injury. So it does seem natural to weigh them against each other.

Thanks for the insight, I was getting worried that the extension conflicts with some other extension I'm using, as I was getting very high bonuses applied. I'd really love to see an option to disable this behavior.