Divider for B29B01W

[Matrix]

I have Adit PMT B29B01W and trying to find an information about correct voltage divider ratio for this tube. Datasheet does not have any information. I have used equal divider with R=10Meg and I have used non equal divider with ratio from the link below and R=10Meg or R=5Meg

https://michaellogusz.blogspot.com/sear ... %20B29B01W

The main issue that I have with the tube is eye-catching non-linearity of the spectrum. The pulse amplifier and high voltage are working perfectly and where tested many times with others detectors, it always +/- linear in the reasonable range. However with B29B01W I have second coefficient a^2 in BecqMoni2011 as 0.00064 that too large. Normally it should be in range of 0.00005 or less.

Any suggestions about the tube? Divider ratio?

Last setup was:
K-30M-D1-5M-D2.....D10-5M-D11-1M-A
D11-100nF-GND
D10-D11 47nF
D9-D10 47nF
D8-D9 47nF

High voltage 500V-800V does not improve linearity.