Why are you even calcing it that way tho.
Wouldn't it make more sense to calc the explosion's width, and divide it by the distance the beam moved on that panel, after all, the explosion itself didn't actually occur till it was hit obviously.
If it was me, I'd take the distance between the rock and the beam on panel (which you seemed to have done already by angsizing the distance between the two rocks?), but from there, given we know the explosion must have covered the whole distance we seen in the time it took the beam to sweep that distance between the two, just divide the beam's movement by that explosion, to get the beam sweep speed.
You seem to be using frozen snail speed stuff, but imo that doesn't check out, correct me if I'm wrong in understanding your calc tho.