OK, it looks like I need to give up on this. I just can't work out what to do. I can't seem to do what the twrp guide says. Then if I ignore that and hope that my TWRP is new enough I can't seem to work out what to actually do. I clearly am not smart enough to understand enough of the words. I will do something else instead. Cheers.

@dad_and_alive Yes, I shall do that. I was hoping there was an app or something built in somewhere to tell you your battery capacity. I have one from lineageos or android or something. Might have just been fun to see numbers.