@chilli It is a number (between 0 and 1000 if I remember correctly) that represents the voltage:
0 for 0V and 1000 for about 3,3V (or the other way round, depends on the "I" parameter of M558). So you would enter something like G31 P500.
But I would try that out / look what G31 (without parameters) exactly tells when it is in the correct position.
You also have to consider that these 1000 steps are the available resolution for the probe input.
So if you map the distance of 10mm to the sensor output voltage range, you actually get a resolution of about 0,01mm at the probe input.
Because of this, maybe the digital output would give you a better repeatability.
I think you should try both variants...
For the probe input: @dc42 may correct me, but I think it shouldn't draw any noticeable current because it is a high impedance input.
There is also a 10k resistor from a RC filter which should limit it to 0,33mA (at 3,3V).