What's the maximum will a soldier can have per rank? How much willpower does an operative gain per rank?

Solution 1:

All rookie soldiers start with 40 Will and gain Will increases as they gain ranks. They suffer a decrease in Will when they suffer a critical wound in battle. Therefore, there isn't really a single low number to worry about, as it varies by each rank. Also, a Squaddie (or any other low ranked soldier) who has been critically wounded can eventually gain enough Will back to be a successful soldier.

The table below shows the low and high Will values a soldier can have, by rank. By checking each soldier against the table, you can determine if their will is too low for the rank they have. I've included the low and high Will values for when the Iron Will upgrade has been purchased. However, I'm not certain how helpful those will be, as you generally purchase that upgrade after your soldiers have been promoted a few times, so you won't expect to see values as high as the columns indicate.

I created the table using the following assumptions:

• Each soldier gets a fixed +2 per level.
• Each soldier gets a random +1d6 per level (iRandWillIncrease=6 in the ini file).
• Each soldier gets another fixed +2 per level when the Iron Will upgrade is purchased (iBaseOTSWillIncrease=2 in the ini file).
• Each soldier gets another random +1d4 per level when the Iron Will upgrade is purchased (iRandOTSWillIncrease=4 in the ini file).

(See http://wiki.tesnexus.com/index.php/XCOM:EU_DefaultGameCore.ini_settings for the ini values.)

                        Iron Will
Rank        L   H       L   H
Rookie      40  40      40  40
Squaddie    43  48      46  54
Corporal    46  56      52  68 
Sergeant    49  64      58  82 
Lieutenant  52  72      64  96 
Captain     55  80      70  110 
Major       58  88      76  124 
Colonel     61  96      82  138

The Second Wave options Not Created Equally and Hidden Potential change all of this. Not Created Equally changes the starting Will from a fixed 40 to a random number between 25-60. Will increases stay the same. Hidden Potential changes the Will increases (and all other stat increases) to be random, so there will be a much larger variation of Will values among your soldiers. The above table can still be used to check whether you have a soldier who is better or worse than "normal", though.

Source.

Solution 2:

According to the wiki:

Soldiers also gain a randomized 2-6 point Will bonus for each rank; if the Iron Will upgrade is purchased this bonus is increased to a possible 4-12 points.

You can further boost this by using some of the Second Wave option, "Not Created Equally", to randomize starting stats:

On one hand a Rookie can start out with a Will stat of 59 and Aim of 80 which is significantly better than the standard stats while on the other hand you can also end up with one with a Will of less than 30 and Aim less than 60 which is substantially worse than standard. Note that this setting does not affect HP, but it does affect Speed - a hidden attribute that is otherwise constant throughout the game.