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References: Ground to air fire (AS, pg 60):
Determine Range
To determine the range between a non-aerospace unit to an
airborne aerospace unit, measure from the edge of the attacker’s
base to the nearest point along the airborne aerospace unit’s
flight path that also lies within the attacker’s forward arc, then add
12 inches to that distance.
If the attacking unit is standing within 2 inches of the aerospace
unit’s flight path, disregard the above measurement rules and
treat the range to the target as Short.
Of note: the rules don't extend to measuring POIs to flight path. And per Homing rounds (AS pg 75) the TAG'd target must be within 34" of the POI, which seems to be impossible for a unit that's not on the board.
Furthermore, the Artillery rules thru Errata doc 2.4 don't ever give eligibility for homing rounds to strike units on the radar map.. in contrast to the rules for Air-to-Air Arrows and Anti-Air Arrows.