1.Tune F3 to A3 and make a wide 3rd, any size.
2.Tune C4 to F3 and make a perfect 5th.
3.Tune D4 to A3 and make a perfect 4th.
4.Tune G3 to C4 and D4. Put the same amount of dirt in the G3C4 4th and the G3D4 5th.
5.Re-tune C4 to F3 and G3
6.Re-tune D4 to G3 and A3
7.Play FC,GC,GD,AD. They should sound the same.
Repeat this exercise using different size F3A3 thirds.