I too would like to know the math behid it, and also would not. If we knew the math, than someon could just make a formula for max efficenecy, in this sense minimum HP penalty, that is if the scaling is not linera.
If it is linear, i dont care that much :D
The factor you see ingame is determined by this formula:
system HP penalty = HpDeductionLogFactor*(max(0, log10(maxHP)+HpDeductionLogOffset))
HpDeductionLogFactor is set to 0.7
HpDeductionLogOffset is set to -5
The log offset basically means when the system penalty is going to apply: -5 => 5 zeros in system HP and the log factor determines how heavy this weighs through.
Example, my ship has 5 000 000 system HP => 0.7 * ( log10 (5 000 000) - 5) => 0.7 * (6.7 - 5) => 1.19
Now if you want to know how this alters your ship. Each block destroyed will deduct => block SHP + block SHP * 1.19 so in this case you almost lose more than double the HP that you gained from that block.