[Avodah] zman hadloko erev Shabbos and motzoei Shabbos

[Akiva Blum]

I wrote: 
<<< If the biur halocho is also using 72 minutes, 
it appears that he is mixing "hours"; those from dawn to tzais for 
plag, and those from sunrise to sunset for 72 minutes. >>>

<<I think I see it differently. The Beur Halacha is saying that Plag is 
not 1 1/4 hours before shkia, but that it is 1 1/4 hours before 
tzeis. That, on its own, is not problematic. But then he calculates 
those 1 1/4 hours as 75 fixed clock minutes, or perhaps 1 1/4 
twelfths of the time from sunrise to sunset, instead of the 
calculation which I would have expected, namely 1 1/4 twelfths of the 
time from alos to tzeis. I do not know why the Beur Halacha chose to do the calculation in 
that manner, but it is undeniable that he did. >>

Exactly my question. Thank you.

However, the BH doesn't say here "72 minutes", only "hour and fifth hour", whatever type of hour that may be.

A & C Walters writes:

<<I really don't understand your chesbon. The Biur Halocho is going with the 
rishonim. (see Rashb"a brochos beis omud alef d"h ulinyan) who says clearly 
that plag hamincha is 1/6 of a mil before shkia (=3 mins if a mil is 18mins) 
Also see Ramba"n in sefer toras hoOdom, that between plag and shkia is 400 
amos, also Tos, Pesochim yud alef omud beis d"h echod, sanhedrin mem alef 
omud beis d"h echod.>>

These rishonim calculate the length of a mil based on from AhS to tzais (which is a 90/96 minute parsa). They call it "hour and fifth of an hour" but that's hours which are from alos to tzais. Their plag is also based on AhS to tzais. My problem is simultaniously using an AhS to tzais day with a plag as 1 1/4 "hours" of haneitz to shkiah.

Akiva