hmmm... trying to figure our spell masteries that improve accuracy with projectiles. Precise casting would help with aim on the missile spell itself, but maybe there are other spells& masteries that generate stackable bonuses? How about some of these, used to augment The Invisible Sling of Vilano (HOH, Societas p38) or other indirect attacks.
In Mercury's Crosshairs
Mu(In)Im 15
Range: touch, Duration: ring, Target: circle
The maga traces a circle around her eye, often using a kohl pencil. Using the guidelines for macrotures from HOH:S pg64, this creates a magical lens over her eye that enhances visual species and superimposes a translucent alchemical symbol for mercury on her sight of a target. The Intellego guideline assures that this enhancement is always accurate enough to negate the penalty for projectile targeting. If the caster exceeds an ease factor of (6+range increment) on a perception+finesse roll, the refined sight grants a further +3 to hit. This spell doesn't need to penetrate the magic resistance of a target.
Because the spell manipulates species around the eye, it distorts the caster's facial symmetry while in effect and causes a -2 on presence rolls. The Columbae Delia of Cornwall is less constrained when casting magical circles. Her version of this spell requires the drawing of a meticulous cat's eye using sharp eyeliner, and the effect dramatizes, rather than impairs, her presence.
(Base 3, +1 touch, +1 ring, +2 circle)
Mastered for precision. A quaesitor could also master for acute senses in case the person viewed is throwing off altered visual species.
Vengence on Aeschylus
InIm15
Enables the caster to see from the bottom of an object that they touch. This helps the caster to target stones dropped from a great height, adding +3 to appropriate finesse rolls. The spell was brought into the Order by Jerbiton who claimed it was first used to murder the playwright Aeschylus via a plummeting tortoise.
(Base 1, +1 touch, +1 diameter +4 vision)
mastered for fast casting
Memoirs of Meneachmus
InIm 5
R: personal D: diameter T:Individual
The spell perfects the memory of a projectile or siege weapon launch, relating it to the caster's studied memories of parabolic arcs and conic sections from the works of Euclid, Eudoxus and Meneachmus. If the caster attempts another such attack under nearly identical conditions --which is usually only possible for a few turns after the initial assault-- he may add his Artes Liberale (Geometry) score to a subsequent targeting or finesse roll on the same target. Alternatively, this spell may be used at leisure during a season to mull over past battles, taking 5 experience points as though practicing finesse.
(+1 perfect memory, +1 diameter, +3 complexity)
mastered for imperturbable casting
The Rainbow Tracer
MuIm 10
The spell changes the audible species of a rock whooshing though the air into colored vivid trails left in the air. These trails indicate both the flight path of the object, and the influence of prevailing winds. If [1] this spell has been cast on a group of stones which are then hurled in a multicast spell, and [2] all the stones have the same target, then the maga can use the motion of each stone to perfect her targeting of the subsequent launches. Assume a +1 to targeting finesse rolls for each stone after the first, up to a maximum of the number of stones launched. The benefit does not carry over to turns after the one when it is cast.
(base 2, +1 touch, +1 diameter, +2 group)
Group seems better than multicast mastery for this one. Possibly 2Xquiet to prepare ammunition but not tip off target until the attack