Hmm... as far as I can see (with a minimal sort of look), it doesn't say. Doesn't say anything about adjusting the soft pedal's effect, that is, unless 6.3.9 Master Volume modifies 6.3.8 Left Pedal Mode's behavior. But I suspect that it must work by subtracting a value from the velocity setting at 6.2.15 or 6.2.16 or 6.2.17 (probably the last two). So the velocity setting or the total loudness might govern the soft pedal's action.
There is a setting where the soft pedal is neither soft nor sos, but changes the velocity of the organ leslie to fast or slow.
Did you try hooking up an expression pedal to it yet? That's kind of fun.
This manual is not the biggest headache of several I'm trying to learn right now--- there are worse examples. I do wish it had been organized a bit better, had gotten rid of unwieldy page breaks that broke up text blocks, which should have stayed together on one page. And since I used up most of a package of markers marking it up for study, it would be nice if Kawai used a paper for the manual that didn't let markers bleed through.
Mine looks like someone has bled to death all over it,,, in six colors.