Thanks TT, I'll contact Kinsler.
My surge tank vent line is -6 which is the same size as feed from the low pressure pump. The fluid velocities in the system are in the normal range so not much back pressure can be expected. I could insert a fixed orifice to make back pressure but the sizing calcs require a lot of assumptions to get the pressure correct and as I said the tank is a pain to get to so a once and done solution with a calibrated relief valve sounds like the right (if expense - kinsler is not cheap) move. The LP pump has a 7 psi internal bypass which is way below test pressure for the surge system so any small orifice might just do the job.
Do you have a check valve in the high pressure side of your system to keep the higher pressure isolated from the low pressure surge tank or is that not necessary?
My solution has the high pressure pump submerged in the surge tank which has a capacity of about 5 liters.