Anti-missile missiles need a warhead strength of 1 to kill an enemy missile. Anything more than that is overkill. 3 warhead strength per MSP is the bare minimum to build an optimal AMM.
Chance to hit is the single most important stat, and is affected by both speed and maneuver rating.
Chance to hit, firing rate, and ammunition limits all benefit from using smaller missiles. 1 MSP is the smallest possible missile.
Minimum range doesn't actually matter, but endurance should be at least 10 seconds. Active sensor range should be longer than AMM range, and MFC range should be similar.
For a size 1 AMM, 0.5MSP engine size is optimal at all tech levels. Always use the highest power modifier available.
Use the minimum size warhead needed to do 1 damage. eg at strength 3 put 0.3334 MSP into warhead.
Agility points are more complicated due to the way rounding works in Aurora, but is readily calculated using the following method:
First, add ~10 seconds worth of fuel. The exact amount doesn't matter.
Calculate (0.5 - warhead - fuel) * Agility per MSP and round down to the nearest half. (ie if you get 3.7, round down to 3.5. If you get 3.2, round down to 2.5).
If the whole number part is even, add 0.0001. (2.5 becomes 2.5001, but 3.5 stays the same)
Divide by Agility per MSP, rounding up at four decimals. ie 0.111101 becomes 0.1112
Put this in as agility MSP.
Fill remaining space with fuel to reach exactly 1.0000 MSP. Aurora will confirm this by showing the missile size in the summary as 1.0000 MSP instead of 1 MSP.
Edit: This information is obsolete. Please disregard.