That is incorrect data, or bad wording. Or, possibly, the actual data depends on a ship class, or game version. Very quick tests for DS in 1.40 show immobilizing values of 26/40 for non-RH non-AE ship; 79/120 for RH non-AE ship and 67/120 for RH AE ship. Note, that Augments value have a huge effect here (as i got you missed it, probably), and that the data not match ">40%" (well, actually it does, in a sense that it is not contradicting it entirely, same way as if ">90%" would be used instead of ">40%", just the actual minimum % of damage needed for immobilization is less than 40), but actually less (setting AE aside). Im sure, that with a proper tests on a "good engine HP" modded ship, as you did in past - you would get the more precise results.A damaged drive leads to partial loss of movement.
If the drive is damaged > 40%, the ship will be immobilized
Ah, btw. Grid is off? Cant believe im actually see it.
Edit: Actually afterthought i did a more correct test, following the idea of immobilizing value simply being a function of a ship's combat speed (no special magic of AE etc). Yes, it is (values above are for a very certain ship builds with a certain load and engines), the faster the ship - the more damage in % to its engine is needed to be inflicted to immobilize it. I guess the actual value are leaning to 50% on a very high speed ships, and to 67% on a very slow ones (again, should be easy to test with some modding tool at hands). So i guess your data (>40%) is actually was quite correct, just it was for some very certain ship builds too.