The amount of dough needed to separate with a blessing: One may only separate Challah with a blessing if he has 1,666.6 grams of flour. If one has less than this amount but more than 1,250 grams he is to separate Challah without a blessing.
 Shiureiy Torah Chapter 3/3-4; This follows the ruling of Rav Avraham Chaim Naah the noted Chabad Posek, author of Shiureiy Torah. So is also the custom of the Sefaradim [see Yechaveh Daas 4/55], and is the vintage custom of Jerusalem Jewry. However some [Chazon Ish] are stringent to require there be 2,250 grams of flower to be allowed to separate with a blessing.
While in Shiureiy Torah he records the amount with a blessing is 1666.6 grams in Piskeiy Teshuvos 242/11 and Hakashrus 14/7 they record in the name of Grach Naah 1660 grams. Seemingly this is a typing error. In “Spice and Spirit” they record 1666.6 grams.
The old Ashkenazi custom: The vintage custom of Ashkenazi Jewry was to separate Challah with a blessing from three Kvartin of flour. [This equals approximately 3 Russian liters which is 1213 grams. If one measures three American pounds then it is 1363 grams. See Koveitz Zalman Shimon p. 77.] Thus they would separate with a blessing from this amount and higher. The source for this custom is Harav Yaakov Viyal in his Sefer Mahriy Viyal chapter 153. There he writes that the Shiur of Challah is a vessel that holds three Kvarton of flour. The Shach [324/3] brings down this opinion and writes that this is the custom. The Aruch Hashulchan [324/10] writes that this is the custom amongst all Jewry and one should not question this ruling.
The Chabad custom: The Tzemach Tzedek [Yoreh Deah chapter 323] writes that “It is known that by us the Shiur of Challah is approximately three Liters [1213 grams]”. This is also recorded to be the practical directive given to people who asked Rav Z.S. Dworkin regarding how much flour is needed to separate Challah with a blessing. [Koveitz Zalman Shimon p. 77] The Ketzos Hashulchan however claims that this Teshuvah printed in the Shut of the Tzemach Tzedek is not truly from the Tzedek Tzedek, as is known that many Teshuvos of other Geonim found their way into the Teshuvos of the Tzemach Tzedek. The Ketzos Hashulchan’s final ruling is as stated above that one is not required to separate from this amount at all and only if there is 1250 grams is it good [but not an obligation] to separate without a blessing. Practically the widespread custom amongst Chabad Chassidim follows the ruling of the Ketzos Hashulchan printed above to separate with a blessing from 1666.6 grams. Nevertheless there are Chabad Rabbanim which rule that one is to separate from three pounds [1363 grams- Koveitz Zalman Shimon p. 77] or 1230 grams [Hiskashrus 731]
 Shiureiy Torah ibid writes one is to separate from 1615.3 without a blessing and less is exempt from separation. However one who is scrupulous separates from even 1,250 grams without a blessing. In however Piskeiy Teshuvos 242/11 they record that one is to separate without a blessing from 1,200 grams according to all opinions. This is the opinion of the Chazon Ish. In Hakashrus 14/7 they record 1,150 grams. In “Spice and Spirit” they record 1,230. Vetzaruch Iyun.