I didn’t specify whether I did it step-by-step or not, so I’m not sure what you’re getting at.
But we might as well make productive use of this comment, so here’s the logic I used:
First all Tuoras use GA, the speed buff of which will start counting down its 3 turns as soon as Deino attacks, and disappear as soon as the third MSS is used. The Tuoras get to use GA once just before the third MSS is used because it’s a priority move, so we get a total of +150%+150%=+300% speed buff just from the Tuoras.
Deinocheirus also gets to use MSS twice, so that’s an extra +20%.
131*4.2= 550.2≈ 550.
Feel free to correct me if I made a mistake.