[GeoffR]

The line spacing scale changed in firmware 3.0.0.

Before 3.0.0 the line spacing slider set the readingLineHeight value in the config to a value in the range 1.3 to 3.0, and was equivalent to the CSS line-height property.

From 3.0.0 on the line spacing slider sets the readingLineHeight value in the config file to a value in the range 1.0 to 3.0, but it is a different scale and not equivalent to the CSS line-height property.

I think readingLineHeight=1.3 in the old firmware gives roughly the same line spacing as readingLineHeight=1.0 in the new firmware. So if you wanted to set the spacing the same as 1.225 in the old firmware, you would need to use a value slightly less than 1.0.

In the new firmware readingLineHeight does not have a lower limit, by editing the config file you should be able to set it to a much tighter spacing than in the old firmware. On my Glo values such as 0.75, 0.8, etc. work and give extremely tight spacing.

Edit: One thing to watch out for if you change readingLineHeight by editing the config file: If you open a book that had already been opened before you made the change, it will not use the global value from the config file but a book-specific one from the database. You can use the KoboUtilities plugin in Calibre to change the line spacing for a book that has already been opened, or there is a line spacing patch that allows you to change the values set by the line spacing slider, which avoids the need to edit the config file.

Edit2: If you're interested in the details of the line spacing slider values, the slider in the old firmware selected from 9 values {1.3, 1.35, 1.4, 1.6, 1.775, 1.9, 2.0, 2.2, 3.0}, while the slider in the new firmware selects from 15 values {1.0, 1.05, 1.07, 1.1, 1.2, 1.35, 1.5, 1.7, 1.8, 2.0, 2.2, 2.4, 2.6, 2.8, 3.0}.