[unknown]
If the registered car has a 75kWh battery, adding 50% would add 37.5kWh
If your friends car only had a 60kWh battery and you wanted to only add 50% then the octopus app would calculate 50% on the size of the register car and try to add 37.5kWh.
So to add 50% to the smaller car you would do 60/75 =0.8 and ask for only 0.8 x the percentage you want.
So with a 75kWh battery car registered and wanting to add 50% to a friends 60kWh car you would ask for a 40% charge. 40% of 75 is 30, which is 50% of the smaller 60kWh battery.
The opposite for larger batteries, so increasing the percentages to reflect.
To be clear it will be against your bill not their octopus bill, but I’m assuming you know this.