The delta in the formula above has been choosed as the delta needed to obtain G(2000V) equal to the Nominal Gain from Hamamatsu (we thus normalize to Hamamatsu Gain our calibration data).
P1 and P2 are respectively the interception with the Y axis and the slope d(LogG)/d(LogHV) (i.e. the power of the dG/G =P2dHV/HV law) of the Calibration line.
By the equation written before we could calculate the working HVL to have a certain gain G0 (generally G0=1x10^6):
To obtain the Error on HVL (=EHVL) you cannot use the above analitical formula (it's too sensitive to decimal digits), but you can use the working formula which takes into account the HVm measured nearest to the HVL:
(Go-Gm)/Gm =~ P2*(HVL-HVm)/HVm
The above working formula involves the HVm measured nearest to the HVL calculated and on the Gain measured at HVL (G0) and at HVm (Gm).