This mod addresses in several ways the time that your wounded soldiers take to heal up.
TL;DR:
1. Recovery time depends on the total amount of health the soldier lost.
2. Percentage of the max health plays a role.
3. No random involved.
4. Shorter average recovery times.
Details:
1. In vanilla the recovery time depends on the health the soldier has in the end of the mission. This makes you pospone ending the mission if you have wounded soldiers and if it's safe for you to postpone ending it until you heal them with medi-kits, because the difference in recovery time will be great. And if the mission ends (due to reaction fire from one of your soldiers, for example) before you have the chance to heal the wounded soldier, it makes you grit your teeth. I think it's excessive micromanagement, which just adds routine, uninteresting actions. Besides, the commander with a wounded soldier would want to finish the battle more quickly, not to prolong it.
Making the recovery time dependent on the total amount of health the soldier lost during the battle takes care of that: if you heal a soldier, it's not gonna reduce the recovery time. Thus, you have no reason not to end the battle when you can. At the same time, the need to heal your soldiers during the battle is not any less: you still don't want them to have low HP, and also the HP they lose when they bleed are also added to the total amount of HP lost. So you still can't neglect your soldiers, but you are not stimulated artificially to prolong the battle until everyone is treated.
2. A soldier with max health 60 is gonna recover from losing 30 HP faster than a soldier with max health 40. The exact values in percent are: less than 20% (20% – 1 HP) lost is healed instantly. Recovery from losing between 20% and (70% – 1 HP) takes 1 day per 5 HP. Recovery from losing between 70% and (100% – 1 HP) adds 1 day per 3 HP. If you lose 100% or more (it's possible if you healed up a wounded soldier for a significant amount of HP and then he was damaged again but not killed), 2 days are added for every HP.
These intervals are additive. Say, your soldier has 50 max health, he first was damaged for 35 HP (15 HP left) and had 7 wounds. You healed all wounds and thus restored 21 HP (his health is 36 HP). Then he is damaged for 25 HP. He still has 11 HP left, so he stay alive, but the total damage taken is 35 + 25 = 60 HP, which is more than 100%. At this moment you kill the last alien and go home. Let's calculate the recovery time.
Less than 20% does not take any time to recover, so his first 9 HP (20% of 50 is 10, and 9 is the first figure less than 10) lost are not taken into account (if he lost only 9 HP, he would be ready for action immediately). Then we add 1 day for every 5 HP lost in the interval 20–70%: (35 – 10) / 5. Then we add 1 day for every 3 HP lost in the interval 70–100%: (50 – 35) / 3. Then, for each HP gone below 100% (plus 100%-th HP per se, since losing this last HP is what actually would have killed the soldier if he weren't healed) we add 2 days: ((60 – 50) + 1) * 2. This makes 5 + 5 + 22 = 32.
3. Any two soldiers with the same max health that have taken the same damage are gonna recover in the same time.
4. Vanilla recovery times always seemed absurdly high to me. This may be "realistic", but I always say that gameplay > realism. If a soldier recovery time approaches 2 months, it is simpler to fire him and hire another one. Notice that with my formula the numbers sharply go up only after taking "deadly wounds", i.e. losing more than 100% health. Until a soldier has taken deadly wounds (which, while possible, is rather unlikely: he is more likely to die), he is gonna recover much more quickly, even considering that in vanilla you can reduce the recovery time by healing him in combat, while with my approach healing, while useful during combat, has no effect on recovery time (other than by preventing loss of HP to bleeding).