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Mixing Kava with soda doesn't seem like the best idea, though if swiy has tried it and enjoys the taste he can use whatever floats his boat.
Check out the Kava Preparation thread for more information on how to prepare Kava. Dose isn't especially finicky with Kava and it varies from strain to strain, not to mention the level of effects desired.
I suggest that swiy starts with a reasonably sized dose (enquirewithin's suggestion is good) and then adjust it as necessary once he figures out what works for him.
Swim's kava must be weak then, swim mixes 8 tablespoons with a liter of water and drink this in an hour. This is a good dose.
That would suggest that swiy's kava is weaker, as he has to use a higher dose. 2-3 tablespoons works for swim for a single dose, with the level of efficacy dependent on the extent of preparation (letting it sit / emulsify for longer helps make it stronger). Occasionally, sublingual ingestion of a tablespoon with subsequent supplement of milk after kava is moist and dissipated in saliva will work well for quick onset and instantaneous yet mild anxiolytic relief (particularly with managing side effects of adderall, including incidence of short-term anxiety and accentuation of obsessive compulsive tendencies on occasion).
The specific variety of kava used, as well as its relative potency, determines the effective dose, not to mention personal physiology and psychology. Trial and error assessments of dose evaluated through self-perception of subjective effects should be enough to gauge an efficient dosage. Don't assume the kava you have is the same as what other people consume, or that it will be the same potency as the previous time, even if using the same vendor / strain.