[Avodah] zman hadloko erev shabbos and motzei shabbos

[Akiva Blum]

RAYWalters Beis Shemesh wrote:

> >The bigger problem is Friday night. According to Halocho, the zman (at least 
   >on Friday) is from Plag HaMincha. This is an hour and a quarter (75 minutes) 
   >before Tteis. (According to the Levush, before shkia). For those who hold 
   >R"T l'iker hadin, it means that plag is only 3 minutes before shkia (as the 
   >BH"l points out Reis Samech Alef, beinyan tosefes shabes). 

On seif beis d"h lehakdim

I don't understand this cheshbon.
Accoring to RT, visable shkiah is irrelavant. It should follow that calculating the length of a 'hour' would include the 72 minutes after shkiah and 72 before haneitz. Total minutes in a day is 864 (instead of 720 acc. To Gra).
One hour is 864/12=72 minutes. One and a quater hours is 72*1.25=90 minutes. So plag is 90 minutes before 72 after shkiah which is... 18 minutes (zmanios, in the winter)  before shkiah.

Where did I go wrong?

Akiva