I never remember the modifiers by heart, but holding Alt, Shift, Ctrl etc works as modifiers for this as far as I remember. Or maybe those modifiers are only for the scroll wheel... I tried to search for the documentation but do not know what search term to use.
Opening Scribus I conclude that when using the scroll wheel:
Ctrl changes in steps by 10
Shift changes in steps på 0.1
But I noticed that for me, clicking the arrows in the box make changes in 0.1 steps and not full 1 steps? Does this mean it's configurable and I have forgotten where I changed it? Or does it depend on the units in use? (I'm using mm.)