If we speak of Agincourt, then there is one thing that is very hard to simulate with Field of Glory. It is infantry packing too dense in muddy field. I guess this happened at Agincourt. French knights exhausted themselves before reaching English lines. If the field would have been a firm grassy plain it would have been totally a different story.
Dense arrow fire forces infantry to advance shields interlocked with ~0,5 meter frontage/man. This slows the advance but makes impenetrable shield wall if the infantry uses big shields. (add the ploughed wet field)
What I have read about cavalry tactics, they all seem to circle somehow around the way how the horses react in battle. Horses are not very brave. They are quite scary pack animals who would like to evade danger when possible. For example, they evade dense infantry formation naturally, because they do not want to run against a wall. Infantry formation looks like a wall for them, if it is solid. Cavalry line charging another cavalryline is propably the same. One of the lines will evade the other. It depends on whose line of horses gets scared first and decides to evade. (cavalry learned to advance with many lines, because if the first line evades, the 2nd will force the enemy line to evade and rescue the first friendly line. Enemy first line is confused after meeting the friendly first line). Fatigue plays a major part when cavalry is skirmishing. The side whose horses gain too much fatigue will start to lose all skirmishes and have lost the battle. The worst situation is when horses after evade cannot outrun the enemy... cavalry often runs away from the battlefield before this happens, to save their skins.
And what longbow should do to cavalry charge? If there are enough arrows, I guess it should look like a thick wall of sticks incoming in front of the horses. Then you add some pain from arrow hits and second and third volley... and there you have a quite a good change of distrupting the charge... actually spoiling the charge by forcing horses to evade.
So in Field of Glory,
I think arrows should have a greater changes to distrupt cavalry. It should be cumulative so that with only 1 longbowman group the changes should be pretty poor, but if you concentrate lets say 4 groups to shoot the same target, the changes should be pretty high. Speaking with familiar wargaming term, the arrows should add SUPPRESSION

You need strong good quality archers, because they need skill to release arrows as volleys and they need skill to shoot enough arrows. There is only a minute or so time to shoot the arrows! What is the time a horse gallops 200-300 yards? If the bowmen are not ready and prepared they do not have time to react... So it is realistic that you have only 1 turn time to shoot at cavalry (1 change).