Question:
We live in USA Our local masjid follows the Fajr and Esha calculation based on ISNA, which is 15 degree for both.
According to ISNA, fajr start time is about 20+ min delayed than MWL calculation method. What is the acceptability of ISNA method and following that for Suhur.
My Answer:
Sharing my personal experience, I had been confused whether to accept or not to accept the "15 degree bellow horizon is suhur" observation result too. And at some point in time I followed it for voluntary fasting. Mostly at around the years when this finding was published, in late 90s.
I was reluctant to blindly take 18 degree, mostly because it coincides with what we call Astronomical Twilight, and guessed maybe classical scholars were reluctant to calculate other angles and took this figure, which was pre-calculated by astronomers.
That was my wrong thought though.
A few years after scholars published their 15 degree finding, another group of scholars with similar understanding carried out their own personal investigation and declared 18 degree was actually correct. 15 degree was too late.
Personally I had been following 18 degree, hence after.
You will find fatwa on this issue here.
http://daruliftabirmingham.co.uk/when-to-begin-fast-15-or-18-degrees/
And long discussion on this issue with history here. Here it's said ICNA has reverted from their earlier claims. And now recommends using 17.5 degree since 2011. Which practically is 18 degree.
https://www.islam21c.com/special/prayer-fasting-ramadan-timetables/
Not to say there aren't any opposing views, such as here.
https://unity1.wordpress.com/2009/09/04/dawn-and-sehrisuhursuhoor-timings-confusion-for-ramadan-in-the-uk-14302009/
But then again Egypt uses 19.5 degree which they more likely came up with, after observation. This angle wasn't pre-calculated.
And the Saudi system of 90 minutes before sunrise also falls within this territory.
Therefore I won't claim someone is in wrong if he follows a community guideline that uses 15 degree. But I personally use 18 degree and it has its backing.