1. Let's imagine that your attack strength is 250.
Now if you fight a legion with 251 remaining health, it leaves exactly 1 remaining health point! However, this legion splinter must still be attacked with many villagers in order to have a 100% chance of victory(see also: [[Combat Probability of Victory|Probability of Victory]]). This is absolutely inefficient and means a huge waste of attack resources!
Remember that the main reason for attacking legions is for the resistance points (RP) earned. The number of RPs earned depends on the health removed from the legion by the attack. Removing health points gives RP equal to the health points plus a factor of 1 to 1.25. (Example: 200 health points gives a maximum of 250 RP - as long as there is no bonus weekend.) So removing the last 1 health point gives 1 RP for this attack.
It doesn't break any rules, but it's unfair to the village owner or guild members who have to waste their resources on it!
As such, it is impolite to attack legions and leave them with low health. There is no official "limit" on how much health should remain. But since the combined The strength of the villagers is well over 200, this legion plus 19 would be a limit value that can very well serve as a very good guide.