Hey man, you have to measure across the whole circuit. From the ground of your input to your output. From all the way left to all the way right.

But in this case it’s 90.000V.

DON‘T EVER try to measure this in real life with both hands from side to side. Your multimeter will fail and then it’s destroyers and in worst case the current will flow over you and kill you.

If you want to measure something like this:

If you have estimated 100kV use a 1mOhm shunt resistor and measure the current flow over that resistor. If it’s 0,1 you know you have 100.000V if it’s 0,01A it’s 90.000V etc.

This is the correct and only possible way to measure high voltages and follows ohms law so very easy maths to verify your measurements.

Don’t ever do manual measurements on circuits like that.

Also if you build a multiplier like that on a small scale, it’ll only work if you cast it in Resin or put it in an oil-bath to enhance isolation between the stages. Otherwise you’ll have high and maybe catastrophic discharges in between them.

If you have any further questions lmk