Rosh Hashanah 13: The Sages Knew Their Measurements

The Gemara's premise have a tradition that grain that has reached 1/3 of its growth before Rosh Hashanah, and is then harvested on Sukkot, is credited to the previous year. But how do you know when it has reached a third of what it will come to grow? But R. Zeira stipulates that the sages were precise and exact and always correct in their measurements. Which is a strong statement, that needs some unpacking. Also, following a sugiah through the daf (the growth of produce for the first third of its growth) including more of R. Zeira's objections (and his appreciation for R. Hanina's retort).