Most folk who add black tea to their wines in order to add tannin generally make the tea in boiling water and add the tea so any bacteria that were rife are now quite expired. But to Doug Brown's question, as with most things you need to suck it and see. In other words, you need to bench test by taking a few equal volume samples of your wine and add different amounts of whatever additive you are adding be it tannin, acids, sugars, spices, etc etc.. and you taste. And you repeat this until you find the sweet spot. Since you know the sample size (and you know the total volume) you simply multiply the amount of the additive that hits the sweet spot by the total volume divided by the sample size.