Well, here's what I reckon. I reckon that Rowling wasn't a fat teenager herself. And I reckon that her older daughter (the baby is too young to be considered) isn't remotely fat herself.
I reckon these things because, when I was 10, I ballooned almost overnight from being quite a slim child into a very fat one."
Last 3 in the house are never the ones I want there, so I reckon it'll be Craig, Derek and Kemal.
Since my American visitors have all, like me, come from the Northeast, the use of reckon is noticeable because it's a word we associate with the Southern US or with rural dialects. Americans tend to think of the British as speaking "better" English, and Americans from the North tend to think of the English of Southerners as being "worse" English. So, if one has those attitudes as background, hearing the word in a British accent can be a little disorient(at)ing.
In a Voice of America interview, Dileri Borunda Johnston, author of Speak American: A Survival Guide to the Language and Culture of the U-S-A, seems to express that surprise:
JOHNSTON: You know, like in England, it's quite common to say 'reckon,' which in American English is quite unusual, or you might here it in the South perhaps or in more old-fashioned contexts."
AA: "Like, 'I reckon I'll go in when the sun gets too hot.'"
JOHNSTON: "Yeah, and people in England say it sort of quite seriously, without meaning it to be funny or ironic or anything like that."
(Johnston goes on to discuss the perils of being an American parent in the UK: "A lot of the grammar is slightly different, so you would have things in British English that perhaps you wouldn't want an American child to learn because it might sound slightly incorrect. Like you wouldn't say 'I haven't got any more.' You would rather an American kid would learn to say 'I don't have any more.'" Gosh, it's hard to be a parent these days, what with drugs and internet porn and variant auxillary verbs...)
The nearest US equivalent to reckon, in most contexts, is figure, as in I figure I'll go for a walk soon. Better Half says: "That sounds sooo American."