In SR, the spacetime interval is given by the metric:
$ds^2=-dt^2+dx^2$ (where I set $c=1$).
To calculate $ds^2$ of a worldline on a spacetime diagram, I measure $dt$ and $dx$ of the line of interest, and plug in the above equation.
But in his answer to this question, user Marek calculated the spacetime interval of the green line as $ds_2^2 = -20^2 + 10^2 = -300$.
This confuses me, because I thought $ds^2_2=-150$. Because to calculate the length of the green curve I have to calculate the length of one part of it and then multiply it by 2, since the two green lengths are equal. By counting from the grid, $dt=10$ and $dx=5$, hence $ds_2^2 =2*(-(10)^2+(5)^2)=-150$.
So is my understanding of how to compute $ds^2$ mistaken or is it just a small mistake from Marek?