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From myzemanim.com for where I live, for this Shabbos, erev Shovuos, the “sho’o zemanis” (Proportional Hour) Gra & Baal Tanya is 1hr:22mins. Nightfall - 3 Stars is around 10:30 pm. Shkia is around 9:20 pm.

How do I calculate the latest time to start Sholosh seudas “sho’o asiris”?

Are there any leniencies bearing in mind the length of the sho’o zemanis?

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3 * 82 min = 246 min = 4 hr 6 min. 9:20 - 4 hr 6 min = 5:14 – Double AA May 23 '12 at 18:20
Can you clarify what you mean by "Are there any leniencies bearing in mind the length of the sho’o zemanis?" – Double AA May 23 '12 at 18:49
Three clock hours = 3 hours. Three sho’os zemaniyos = 4 clock hours approx. (1) Do we really need to wait 4 clock hours? (2) We can't eat until after nightfall (and maariv). This is (at least) another 70 minutes later. – Avrohom Yitzchok May 23 '12 at 19:28
I don't see how either of those will lead to a kula in terms of seudah shelishit. But you should CYLOR regarding if you can say maariv and/or kiddush before tzeit as there are some kulas to be found in that area. – Double AA May 23 '12 at 19:31

The start of shaa asiris is nine twelfths (or, more simply, three fourths) of the way from sunrise to sunset. Subtract sunrise from sunset (so if sunrise is 7:41 and sunset is 18:12, you get 18:12−7:41=10:31), divide by four (10:31/4=2:37:45), and subtract that length of time from sunset (18:12−2:37:45=15:34:15).

That said, this is a math-oriented answer, not so much an halachic one. I can't speak to your subquestion, "Are there any leniencies bearing in mind the length of the sho’o zemanis?".

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Why didn't you use the numbers in his example? – Double AA May 23 '12 at 18:38
@DoubleAA he gave no sunrise time in his example. – msh210 May 23 '12 at 18:39
@DoubleAA PhDs in math are notorious for their poor basic-arithmetic skills. :-) – msh210 May 23 '12 at 18:47
@DoubleAA 5:13 not 5:14. (Rounding error.) – msh210 May 23 '12 at 18:51
As they say 'Do not rely on zmanim times to the last moment' – Double AA May 23 '12 at 18:54

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