How can someone figure the Gra’s tzas hacochovim?
Answer:
According to the Gra a mil is 22.5 minutes (32 mil between hanetz and shkiya: 32/12*60=22.5).
The time of tzeis hakochavim according to the Gra is 3/4 of a mil, which is some 17.5 minutes after shkiya.
This is true for an equinox day, and the time changes around the year.
For de’raisa halachos, it appears however that the Gra agrees with the decree made by the Geonim to wait for the appearance of smaller stars, and the time in practice will therefore be later.
Best wishes.
Just thought I should point out that 22.5 times .75 equals 16.875, i.e. a bit less than 17, and not 17.5.
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