Tool for Measuring Low Current
The only sure fire way of measuring low current on a high-voltage line is with differential probes rated 80 KV (4 each 500M 20KV resistors, Caddock 0.025%) each across a precision (1%) 1K 100W-300W resistor which is in series with your load (the high wattage prevents arcing across the resistor). The source common mode voltage of 60KV is divided by 10,000 to get a safe 6 volt common mode voltage.

10mA would give you 10 volts across the current sense resistor and 1 mV at the TL052 buffer inputs and 10 mV at the AD524 inputs.

The AD524 gain is set to 1,000 and can be read with any DVM at the output pin as 1 volt/1 mA.

The AD524CD runs on +/- 15 volts. Because you are dividing the input by 10,000 the input impedance of the AD524 is an issue. I added a TL052 dual op-amp wired as a buffer with a gain of 10.00, to the AD524 inputs. The AD524 also has input and output offset trim and fine-gain trim.

Recommend using battery power to avoid common mode errors but safe for PC running LabVIEW if accumulating results.