Reference for Mikvaot 6:11
הַמַּטְהֶרֶת שֶׁבַּמֶּרְחָץ, הַתַּחְתּוֹנָה מְלֵאָה שְׁאוּבִין וְהָעֶלְיוֹנָה מְלֵאָה כְשֵׁרִין, אִם יֵשׁ כְּנֶגֶד הַנֶּקֶב שְׁלשָׁה לֻגִּין, פָּסוּל. כַּמָּה יְהֵא בַנֶּקֶב וְיִהְיֶה בוֹ שְׁלשָׁה לֻגִּין. אַחַד מִשְּׁלשׁ מֵאוֹת וְעֶשְׂרִים לַבְּרֵכָה, דִּבְרֵי רַבִּי יוֹסֵי. וְרַבִּי אֶלְעָזָר אוֹמֵר, אֲפִלּוּ הַתַּחְתּוֹנָה מְלֵאָה כְשֵׁרִים וְהָעֶלְיוֹנָה מְלֵאָה שְׁאוּבִין וְיֵשׁ בְּצַד הַנֶּקֶב שְׁלשָׁה לֻגִּין, כָּשֵׁר, שֶׁלֹּא אָמְרוּ אֶלָּא שְׁלשָׁה לֻגִּין שֶׁנָּפְלוּ:
Regarding the purifying pools in a bathhouse [used for rinsing off in cold water after a warm bath], where a lower one is filled with drawn waters and an upper one is filled with valid [waters], if there are three <i>log</i> opposite the opening [connecting the two pools], it [the upper pool] is invalid [due to the three <i>log</i>, if it contains fewer than forty <i>se'ah</i>]. How far must the hole span in order for it to contain three <i>log</i>? One three-hundred-twentieth of the pool [i.e. since three is one three-hundred-twentieth of 960, which is the number of <i>log</i> in forty <i>se'ah</i>], according to Rabbi Yose. Rabbi Elazar says: even if the lower one is full of valid [water] and the upper one is full of drawn water, and there are three <i>log</i> opposite the opening, it is valid, for they [the Sages] only said "three <i>log</i> that fell" [into a incomplete <i>mikveh</i> invalidate it; and such a case is not considered falling in].