By checking each soldier against the table, you can determine if their will is too low for the rank they have. The table below shows the low and high Will values a soldier can have, by 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. Therefore, there isn't really a single low number to worry about, as it varies by each rank. They suffer a decrease in Will when they suffer a critical wound in battle.
All rookie soldiers start with 40 Will and gain Will increases as they gain ranks.
