This is a Halachic custom based on the fact that there are many opinions how to measure the exact amount of flour which requires one to take off Chalah. We therefore take off Chalah without a Berachah on the amount that is given by the opinions giving small amounts (according to Sefer Hakashrus 14:7-8 about 2.3 pounds (1.05 kilo) of flour), and take off with a Berachah by opinions that give amounts that according to most opinions would be obligated in Chalah (this amount varies from 3.652 pounds (1.66 kilo) to 4.95 pounds (2.25 kilo).

All the best,

Yaakov Montrose