This what I use for my single key diatonics. The fifths are almost perfect (+2 as opposed to +1.96). The thirds are significantly flatted (-8) . The intervals for IIm chord (A minor in this case) are compromised but minimally so.

Please note: that in order to use this temperament, some open 'harp surgery is required. You must have two separate strings dedicated to the second step in the scale. Most diatonics, even single key , do not. Then you have to cut the felts for your chord bars so that the IIm, and partial II7 if you have one, are open for the lower pitched #2 string only and the V and V7 are opened for the higher pitched #2 note only.

WILL SMITH PERFECT FIFTH TUNING [key of G]

STEP                                       NOTE                             CENTS

   1                                                g                                       0

 

  2 high                                      +a                                    +3

  2 low                                       -a                                     -5

 

   3                                                 b                                     -8

 

   4                                                 c                                     -2

 

   5                                                 d                                     +2

 

   6                                                 e                                     -8

 

   7                                                 f#                                    -8

 

 

Obviously, these values can be transposed to any single key.  Keeping the #1 step at zero means that the other steps in the scale surround it from top and bottom, so if you are playing with other instruments, your overtones are pretty much covering the wholeness of the key, and resolving satisfactorily on the I tonic/home chord.