Digital BW, The Print

For some time now I've been using QTR for my black an white prints. 
I've been reading Tom Moores excellent User Guide with great interest.
One thing puzzles me though. In figure 11 – Table of Ideal Desities, 
Tom suggest, that with increasing dMax the print as a hole shall print 
darker and darker, So a 50% grey will have a density of 0.568 with a 
dMax 1.60 paper, and much darker, 0.711 with a dMax 2.4 paper. Wouldn't 
it be more logic to balance the curve with a constant midtonevalue?? 
And what should that value be?