There’s been a lot going on in these parts, so you’ll have to forgive me in being late in passing along some correspondence. Shortly after I posted on “reckon,” Wes Davis, a longtime friend of this blog, sent along an article from the Atlantic and commented, “David Frum [the author] helpfully underlined the NOOB for you!” The relevant paragraph:
Of course, the underline wasn’t for emphasis but to indicate a link. If you want to follow it, click here.
notoneoffbritishisms.com 
Yeah, but Frum gets to … because he’s Canadian. In Canada people say “orientate,” put a “u” in color and labor and harbor, call powdered sugar “icing sugar” and use umpteen other British locutions.
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That’s probably because they didn’t lose the War of 1812.
Here’s the Ngram graph for the use of ‘reckoning’ in the UK and the US:
https://books.google.com/ngrams/graph?content=reckoning%3Aeng_us_2019%2Creckoning%3Aeng_gb_2019&year_start=1800&year_end=2019&corpus=26&smoothing=5&direct_url=t1%3B%2Creckoning%3Aeng_us_2019%3B%2Cc0%3B.t1%3B%2Creckoning%3Aeng_gb_2019%3B%2Cc0#t1%3B%2Creckoning%3Aeng_us_2019%3B%2Cc0%3B.t1%3B%2Creckoning%3Aeng_gb_2019%3B%2Cc0
And here’s the Ngram graph for the use of ‘a/reckoning’ in the UK and the US:
https://books.google.com/ngrams/graph?content=a%2Freckoning%3Aeng_us_2019%2Ca%2Freckoning%3Aeng_gb_2019&year_start=1800&year_end=2019&corpus=26&smoothing=5&direct_url=t1%3B%2C%28a%20/%20reckoning%3Aeng_us_2019%29%3B%2Cc0%3B.t1%3B%2C%28a%20/%20reckoning%3Aeng_gb_2019%29%3B%2Cc0
Any ideas as to why they should look so different, anybody?
I always thought the most formal use of the verb ‘to reckon’ is ‘to count’ or ‘to calculate’. The example here seems to be using it in that sense.
As in a “ready reckoner”, a table of calculations..
I remember it was a popular word on the 1950’s cowboy show The Rifleman.