So, I went out and did some testing on Drain II and Drain III. I tested on Plateau Hares in Abyssea La-Thiene due to them having 0 MDT, with 0 Drain/Aspir Potency, 0 Dark Affinity, and on days other than Darksday/Lightsday. The results are sorted from highest to lowest with the bracket for easier reading.
Drain II, 460 Dark Magic Skill
616
611
605
604
603
601
592
587
565
563
563
557
551
548
543
541
540
532
523
516
507
502
501
491
481
460
443
435
434
430
Drain II, 520 Dark Magic Skill
684
679
678
677
673
672
668
663
663
657
652
645
616
606
596
594
593
583
570
560
540
532
524
511
505
501
500
498
493
491
Drain III, 460 Dark Magic Skill
843
835
828
819
813
805
799
796
778
778
767
762
751
728
726
723
719
707
701
693
690
684
678
677
669
657
656
651
647
646
Drain III, 520 Dark Magic Skill
935
928
925
924
918
908
885
882
881
875
874
868
868
850
842
820
816
811
793
775
766
757
755
754
753
752
738
734
732
708
From these results I can make the following conclusions:
At Dark Magic Skill levels relevant to knowing Drain II and Drain III, the base potencies are (plus or minus a couple points due to small test samples):
Drain II: Dark Magic Skill + 165
Drain III: Dark Magic Skill * 3/2 + 105
It is likely that they have different formulas at much lower Dark Magic Skill values (like with Drain's formula), but as I have no way of testing it (and it's not useful information since Dark Knights should always have high enough Dark Magic Skill for it to not matter), they are not relevant to this test.
Drain II has a random variance between the base value and ~70% of the base value (might be 2/3s, more testing samples would be needed).
Drain III has a random variance between the base value and 75% of the base value.