Here for a 4 key chromatic Bb/F/C/G

This is actually what I use most of the time on the Deleter Chromatic, which is centered around keys of F and C, with peripheral keys of Bb and G used if a given melody modulates there. This would also work reasonably well for  a standard 21 chord Oscar Schmidt (although an E7 chord would be out).

g            0

g#        +2

a             -5

a#        +2

b          -5

c            0

c#       -7

d         -2

d#      +3

e         -5

f            0

f#       -7


You might notice that the d# is only moved one cent away from the a#. This is not hugely significant, I just want to keep the total high/low divergence from zero to about 10 cents.

If a song has a prominent B7 chord, I might re-tune for those notes (b-d-f#-a) to make them blend a little better. Or if the G# diminished seventh is important in a tune, I might bring the g# down to zero for a tighter fit. The dim 7ths, which are built on minor third intervals, are tricky to sweet tune, but since they are generally used to create a tension which is almost immediately resolved, being a little more "out-of-tune" is less of an issue.