Dump is important and should be selected so that you do not fall out of chair and shear stress on skin is also avoided.

Christopher Reeve was told by his doctors not to travel, but hid did and he died. What actually killed Christopher Reeve:
http://sci.rutgers.edu/forum/showthread.php?t=35839

J Biomech. 1995 Feb;28(2):225-30.
"Design criteria for the reduction of shear forces in beds and seats."
R.H.M. Goossens, C.J. Snijders.
http://linkinghub.elsevier.com/retri...21929094000526

Has anyone read this or have access to it? I have only read the abstract. What is meant by the quoted bits? Am I on the right track?

For chairs it was found that when little shear is accepted, a fixed inclination _between_seat_and_backrest_ can be chosen between 90 degrees and 95 degrees.
Code:
90...|
95°  |
_____|
0.../  This angle is known as dump in wheelchair mechanics.
5° /   It is expressed at a difference of seat height at the
       front less seat height at the back.
This is required to prevent the user falling forwards. At the same time decubitus ulcers must also be avoided.
For beds a parabolic relationship was found between seat and backrest inclination with a maximum _seat_inclination_ of 20 degrees at a _backrest_inclination_ of 50 degrees.
Code:
      A
   __110°/
B 20°\  /50° C
   ___\/___
A fully adjustable chair would therefore have this range of adjustment.
Code:
90...|
110° |
_____|
0.../ 
20°/
A parabolic mechanism is difficult to construct, but a linear mechanism would be a simple 2:1 reduction. I.e. reclining the backrest (C) from 90 to 50° results in inclining the seat (B) from 0 to 20°. A cycloidal mechanism could also be constructed if a close approximation of a parabola is actually required.
Code:
  ----+-----+-----+-----+-----
A  90 |  95 | 100 | 105 | 110 _between_seat_and_backrest_
  ----+-----+-----+-----+-----
B   0 |   5 |  10 |  15 |  20 _seat_inclination_
  ----+-----+-----+-----+-----
C  90 |  80 |  70 |  60 |  50 _backrest_inclination_
  ----+-----+-----+-----+-----
     seat              bed