What do you mean with "calibration area"?
It is the size of the area that a projectile will hit with 100 marksmanship. Different projectile types (small arms, cannon/howitzer, mortars) have different calibration areas.

- Are you planning to introduce ammo limits on artillery? This would influence balance as well!
No.
- With the changes to cavalry (more powerful), are you planning to make them somewhat more expensive to keep the focus on infantry?
Costs are definitely being looked at.