Thought on things that effect missile accuracy:
These are just thought, some effects are below the granularity of TROS.

Taking a shot: with your senses collect data about all the variables that will affect your shot, use your experience to judge how you should aim to hit the target, use your hand-eye coordination to tell your body what to do, use your gross motor control to keep your arms steady and "stay on target, stay on target".
Eye-sight and touch (for wind) are your senses most used.
Making judgements uses aspects that make up your mind (3 dimensional analyses primarily, but other stuff too) and your experience firing the weapon.
Eye-sight and touch: PER
Brain power for making judgements: Proficiency and WIT or MA or PER I'm not sure, possibly a combination?
Hand-eye coordination: AG
Gross motor control: STR and AG (keeping a longbow steady as you draw back the arrow requires great muscle control, then the final aim after the draw is stressful too). Gross motor control can be done entirely by the body or the shooter can use a brace like a tripod, or leaning against a wall, or laying a crossbow on a rock before firing to reduce inaccuracies caused my a moving shooter. Hand and arm attachments can help too.
WP, TO and EN may have effects too but I think they have impact below the granularity of the system.

Some causes of inaccuracy are constant and some are variable.
A constant cause (like range, or a steady wind) can be perceived, judged, and accounted for.
Variable causes (like gusty winds) can only be mitigated by the characteristics of the flight, judging won't help because the variability can't be accounted for. Example: faster and heavier missiles are more accurate in gusty winds, because the wind has a lower effect on them, William Tell would not be able to adjust his aim to take into account a variable wind any better than I. But he would be better at estimating range and adjusting his shot accordingly, he would also have other missile weapon use advantages over me, my advantage is that I'm alive so actually I could probably beat William Tell in crossbow firing match. : )
Magic and Guided weaponry can reduce the inaccuracy no matter if the cause is variable or constant.

Wind has a constant aspect and a variable aspect
Rain has a constant aspect and a variable aspect (it tends to be less variable than wind).
Range is a constant cause and can be accounted for.
Target movement can be fairly constant if he is charging at you (or at lease predictable, for instance constant acceleration like a falling object).
Or it can be random
Shooting from a moving platform (like a horse) has constant and variable effects on accuracy.
Bow quality has a constant (ie. it always hooks) and a variable (degradation and others) effect on accuracy.

Air resistance and time of flight reduce energy imparted on the target, and thus damage
Single missiles with long time of flights, that can be seen by the target who has good enough eyesight and with room to move can be dodged. On a battle field this is rare, except for skirmishers which is what PCs tend to be.

On another note: something I'd like to see addressed in the Flower of Battle is how shields had a major benefit when you are in a close formation facing massed missile fire, especially long range missile fire. They werent as effective facing skirmishers and light cavalry with missile weapons who get in close and have very low time of flight so you don't have a chance to duck and cover.

What I'm really working on is firing into melee, this is something that requires much consideration.

Everything heavily in my opinoin.

Likely someone might say turning these ideas into a system would be terribly complex and boring to play, people have said the same things of making a simple and realistic hand-to-hand system. But I think we have fairly good proof that that is possible. I also believe that making a simple and realistic hand-to-hand system is MUCH harder than a simple and realistic missile system. Jake deserves a lot of praise.

Another note: the missile system in TROS is great!

Scott