A way to measure the amount rather than guessing at it, since "start with a centimetre and a half" is a reasonable opening bid and not an answer.
On a toile, before you adjust anything, draw a horizontal balance line across the back at hip level, parallel to the floor when the pattern is flat. Put the toile on and look at that line in a mirror.
What you will see is the line dipping or rising at centre back rather than staying level. Pin out the fold until the line is horizontal again, and the amount you pinned out at centre back is your adjustment. Measure it, take the toile off, and transfer that number to the pattern.
The reason this beats trial and error is that a fold of fabric is very hard to judge by eye, it looks like the same amount of excess whether it is one centimetre or three, and you cannot see your own back properly anyway. A ruled line is unambiguous and it shows up in a phone photo, which is how you look at your own back honestly when sewing alone.
Same trick works for the front, and for shoulders. Balance lines are the single most useful thing I picked up in fitting, and almost nobody draws them because they look like extra work on a garment you are going to throw away.