here's one innovative and scientific way to tune your tambura to perfection: (for the physicists, it is based on the phenomenon of beats.)
1) To ensure perfect sync betwn the two Sa strings:
measure the length of the strings of the tambura and mark off the midpoint(approx.). Now strum the two Sa strings simultaneously and at once touch (mind you only touch,not pluck again or forceful touch which will simply cause the strings to stop vibrating)--- by doing this you are ensuring a nodal point at the midpoint. Now place your ears at the 'kodam' of the tambura. You will be able to hear the Sa of the next octave. If the strings are perfectly tuned, there will not be any discernible modulation i.e. there wont be any Beat. If however you are able to make out some modulations, you simply need to tune better and check again by following the above procedure once more.
2) to ensure perfect sync betwn the mantram (the lower Sa string)string and the above Sa strings:
Follow the same procedure for the mantram string as mentioned above. Only in this case do not touch the Sa string so that you will hear Sa of the same octave. Again if there are any modulations(i.e. any distinct maxima and minima in the frequency) your tuning is not perfect.
The procedure can be repeated for higher octaves also but in this case the sounds may not be intense enough to be discernible. The procedure, if employed, will only take you a step toward attaining more Shruti Shuddham. A physics professor told me this with respect to a sitar and i tried it on the tambura.Surprising how principles from physics can be used effectively to attain perfection which is ultimately any musician's goal.
(the physics behind this is quite simple but i will post it if anyone is really interested...)