[T]he twin boosters generate 3.6 million pounds of thrust each…. The pair will provide more than 75 per cent of the total SLS thrust at launch.
In the core-burning solid rocket motors (which I believe is very much most of them), adding 25% to the length adds 25% to the thrust, approximately, unless you change the shape of the interior to make it burn faster or slower. Since 3,600 klbf is very nearly 125% of your 2,800 klbf, it sounds like it's the same shape inside.
Or, you could've just skipped the working out, and taken their 75% figure at face value. OK, I just might like working it out myself and double-checking, too. Assuming their figures for booster thrust and yours for SSME are correct, it's actually 83% booster, which is indeed 'more than 75 per cent,' but I would have thought was worth using something closer, like ⅚ of the thrust.