NPSH Test Translation according to HI 14.6

[HARI KRISHNAN]

Hi,
Hydraulic institute 14.6 edition 2011 says
In clause 14.6.6.1.1 Translation of test result into data based on the specified speed of rotation (or Frequency) and density, says
(NPSHR)T =(NPSHR)(nsp/n)^x
Where,
(NPSHR)T NPSHrequired during test
NPSHR Actual value of NPSH
nsp Specified speed of rotation
n Test RPM

The clause states clearly that the value of x shall be 2 if the specified condition which are stated in the clause 14.6.5.7.2 for the speed of rotation and the rate of flow are satisfied.
But, the arbitrary value x varies between 1.3 to 2 when the pump deviates from the clause 14.6.5.7.2 due to the thermodynamic effects, variation in surface tension etc.,
Question is on what basis the arbitrary constant is agreed to be between 1.3 to 2 and why most of the pump manufacturers choose the value 1.5 for the translation of result.
Does this arbitrary value 2 is derived from the affinity law where the head is proportional to the square of the speed?
AS a Budding Engineer i need a clarification on this.