The Shulhan Aruch in Siman 175 details the conditions necessary for reciting the Beracha of "Ha’tov V’hametiv" when drinking an additional glass of wine that is superior to the first. The Poskim discuss whether there is a minimum amount of wine that must be drunk in order to recite this Beracha. With regard to Hagefen, drinking even a minute amount of wine requires a Beracha. With respect to "Ha’tov V’hametiv," logic dictates that one must drink a significant amount, since the whole purpose of the Beracha is to offer praise for the abundance of two kinds of wines. Accordingly, the Kaf Hahaim (Rav Yaakob Haim Sofer, Baghdad-Israel, 1870-1939) rules that one must drink at least a Revi’it (3 oz.) from both the first and second glass. Hacham Bension (Or Lesion vol. 2) rules that it is the second glass which demonstrates the abundance, and therefore one must drink at least a "Mulo Lugmo" (cheekful) (preferably a Revi’it) from the second glass. The first glass has no minimum requirement. Hacham Yosef Yedid Halevy (1866-1930, Aleppo-Jerusalem) rules that there is no minimum amount for this Beracha. Even if one drank just a sip from both glasses, he can recite the Beracha of "Ha’tov V’hametiv." The fact that he has two different types of wine is enough to warrant the Beracha. Hacham Ovadia, in Hazon Ovadia, rules in accordance with this opinion. This is very relevant, since in many instances, a person only drinks a sip of the Kiddush wine and then a fine bottle is served. The fact that he only consumed a sip does not nullify his ability to make this Beracha.
Some authorities rule that in order to make "Ha’tov V’hametiv," the first wine must still be available, when the second wine is consumed. If the first bottle is depleted before the second bottle is drunk, the abundance is not apparent, since the second bottle was served out of necessity. Maran disagrees and holds that even if the first bottle is finished, one may still recite the Beracha. Nevertheless, Hacham Ovadia rules that this disagreement generates a Safek Berachot (uncertainty with regard to reciting a Beracha) in which case the Halacha is "L’hakel"-to be lenient and refrain from reciting the Beracha.
Recently, the following question was asked of Hacham David from the Daily Halacha Shiur: What is the Halacha if the first bottle is depleted, but there is another unopened case of the same wine in the basement? Does the fact that he has access to the same wine, and nevertheless choses to drink an even better wine, warrant the Beracha of "Ha’tov V’hametiv?" He responded that the Halacha requires the actual first bottle to still have wine. The fact that he has reserves is not sufficient to mandate the Beracha.
-Even if one consumed only a minute amount of wine from both glasses, he can still recite "Ha’tov V’hametiv."
-If the first bottle of wine is finished before the second bottle is served, the Beracha of "Ha’tov V’hametiv" is not recited."