The Gemara on Shabbos 35a says:
אמר רבי חנינא: הרוצה לידע שיעורו של רבי נחמיה יניח חמה בראש הכרמל, וירד ויטבול בים ויעלה, וזהו שיעורו של רבי נחמיה.
Anyone who wants to know the shiur of R' Nechemia's bein hashemashos should leave the sun at the top of Mt. Carmel, go down, dip in the [Mediterranean] Sea, and go up. This is R' Nechemia's shiur for bein hashemashos.
Various Rishonim have different explanations of what this means. For instance, Rashi says the time it takes is the length of bein hashemashos, while Tosfos says it only identifies the beginning of bein hashemashos, and Ramban includes the time it takes to go back up to the top of the mountain.
Most of the Rishonim never saw Har Hakarmel and were working from their understanding of the Gemara. Has anyone tried it from different places on the mountain to see if/how it works out according to the explanations of different Rishonim? Maybe one of the yeshivot with a heavy Land focus?
(If there's a record of the Ramban himself trying it after he moved to Israel, that's even better!)