The KL usually listed as "standard" is 3rd string C lowered to B. That gives a useful minor, I have it on RKL.
The next one I would recommend is to raise the same string to C#. This gives a 10 string 7th (I have G on top) when combined with (standard) pedal 8. On my guitar it's RKR.
For interest, my other two (central levers) on C6th are CKL raises both As (the 6ths) to Bb, making a C7th but mostly used for single note passages rather than in chords. The other (CKR) is a "reverse" pedal 6 (lowers 2nd E to Eb and raises 6th E to F. i.e. the opposite functions to pedal 6). Not often used.
Pedals 5, 6, 7 and 8 are all used pretty often but the one change hardly ever used is pedal 4 which raises both A's to B. Since pedal 7 also raises A's to B (admittedly with other changes too) I hardly ever seem to find a need for pedal 4; also it's quite a shift from the "home" position of the foot (in my case, over 6 and 7).
Maybe 4 needs to be an E9th pedal and put the A's change on a vertical.
