A sector’s inner grid has 7x4 + 4 = 32 planets in its defense perimiter. These planets protect a 7x7 grid with 60% spawn chance = 49 * 0.6 = 29.4 planets.
Controlling a defense grid should allow you to not defend inside the grid. To accomplish this, each should provide 30.4x the defense effectivity of an ordinary planet. 3 neighbours offer a 3.375x multiplier to your defense perimeter, leaving a 9x multiplier.
A 9x multiplier is overly strong for initial conquest purposes, so half of this multiplier is transferred to reduced supply usage.
But we also need to compensate for the much larger rewards available from pillaging inner world supplies and infrastructure. This value is abstract and will vary over time - we attach a bit more than double-value to this in the form of halving supply costs again and bumping defense multiplier to 5x.
By making the Deutonium multiplier 0.3x we make defense planets the most attractive supplies / Deut in the game once developed - giving them some additional standalone value.