The speed of flashy flash must be recalculated.
In fact, if you look at the scene, the stone moved noticeably.
The stones expanded to the same level as the width of bridge before they started fighting in earnest, but after that you can see that they have expanded more than the width of bridge.
I think it will probably give consistent results.
The speed of casual saitama consistently equivalent to flashy flash without kill move technology
By the way, why not use flame's blast speed in the calculation?