Blood in flight is just a projectile. A drop leaves the source, arcs under gravity while air drag bleeds its speed, and strikes the wall — the same physics as a thrown baseball.
The shape it leaves is honest. The ratio of a stain's width to its length is the sine of the angle it struck at: sin θ = width ÷ length. A near-round stain means a steep, near-90° hit; a long thin one means a shallow, glancing hit — and its tail points the way the drop was going.
Run a line back along each stain's long axis and the lines cross near where the blood came from — the area of convergence. A least-squares fit of many strings lands it within centimetres.
The honest limits. This bench models a vertical slice of the scene, so the origin it recovers is the two-dimensional area of convergence — reliable. Straight strings quietly ignore that real drops travel on curved arcs, so heavily-arced throws, too few strings, or near-parallel stains blow the estimate up. That is exactly where over-confident courtroom testimony has gone wrong. Trust the physics; test the story.
Something in the simulation stopped unexpectedly — the lesson continues without it. Nothing you did was wrong; you can move on.