QTR-Quadtone RIP

Printing the ink pattern in calibration mode to determine the ink limit 
(Epson 1400 Claria Black for BO at 2880dpi) I often get a reading with a 
shape like this:

   0  0.03
   5  1.08
  10  1.63
  15  2.00
  20  2.15
  25  2.22
  30  2.22
  35  2.21
  40  2.21
  45  2.21
  50  2.21
  55  2.22
  60  2.23
  65  2.23
  70  2.23
  75  2.22
  80  2.21
  85  2.21
  90  2.20
  95  2.20
100  2.21

What does one do in such a case?
- ink limit of 25 or 30, because D drops afterwards (although it gets to 
2.23 much later), or
- ink limit of 60 ?

Also: is it normal that I get a lower Dmax at 2880dpi (2.23) than at 
1440super (2.27)?

Thanks, Louis