Well, I suspect that the det curves are NOT linear, because in my tests a SL=50 missile was always detected at a range corresponding to det curve value of 5, not 50!
But this missile was really probably 40 (front aspect -10) and detection was affected by low altitude and sea state.
Anyway it's quite confusing and I plan extensive tests that will clear this all definitely (starting at high alt, measuring aspect effect, plotting det curve value/SL dependance, measuring effects of low alt and sea state for sea skimmers, combining it all together).