That's just the thing, though. Depending on how you hold your hand, the answer would be different. On my bass, directly across from my thumb when it hits the heel, the note is a D. However, since I normally don't shape my hand that way, it's actually easier to find the Eb than the D. I just checked, and if I were deaf and just placed my thumb at the heel and placed my index finger where it would usually go, I land on the quarter tone between D and Eb every time.
I may be crazy, or have faulty technique, etc.... but without the ear being in charge of everything I still find the notion of D or Eb necks kind of spurious. Sure, the landmark gets you in the ballpark, but even the slightest variance of string length at the bridge, or the slightest variance of technique regarding hand position would render the question moot for in any real sense that involves exact intonation. I agree with
@Neil Pye that after a few minutes of range finding, a good player should be able to find the notes they are looking for both aurally and kinesthetically.