Once you have the linear regression parameters you can calculate the voltage based on an empirical formula you'll find out. Then just one measurement at the HV test point is needed.
1. You'll have to measure the voltage at one of the HV test points = V_HV
2. and at the "+" terminal of one of the GM tubes. = V_GM+
3. Then repeat this over the whole voltage range by changing RV1 on the HV board every time.
10-15 points are enough. You can have your own empirical formula to measure the high voltage with very basic equipment.
Warning: clear the stats memory before running this, since this does the linear regression, too. -> Clear 4
A = V_HV
B = V_GM+
C = Z_output (Y reg)
D = V_0 (X reg)
A001 LBL A
A002 INPUT A
A003 INPUT B
A004 RCL A
A005 RCL B
A012 RCL A
A013 RCL B
A018 STO C
A023 RCL A
A025 STO D
A026 RCL A
A028 RCL C
A029 RCL D
Then just check <- L.R. for the regression parameters.
The write up of the measurement is coming soon.
SPOILER ALERT!!! here comes an example formula:
V_0 = 1.555 * V_HV - 48.64