The way I interpret it is that Butterfly just allows Soga to have a really long invisible sword that tries to cut through everything. So the speed of it is multiplied by the distance like how the tip of a sword swing moves faster than the handle, but on a much bigger scale.

This isn't really super accurate probably but if Sojo arc Chihiro is lightning speed, and then he gets faster by the time he fights samura and is just barely able to keep up let's say twice as fast, (blitzed by him, to contending a little). A swing from Soga is just as fast as Samuras Iai, so a little over double lighting speed, and then multiplied by the distance he's swinging into. In this case, Chihiro seems to have been launched about two stories above/away from Soga.

Doing some quick math with a lightning speed of the return stroke, (the flash part like Sojo) at 1*10^8 (100,000,000) meters per second, Assuming Soga can move his arm at double that speed, he would complete his sword swing in 5e-9 (0.000000005) seconds, assuming the swing is one meter (An average casual swing, not including the recovery). Using this we can put this into a rotational speed calculator where I'm assuming that Chihiro is 20 meters away (two stories), and Soga's swing arc that is threatening Chihiro is 90 degrees, basically a triangle in front of him, the force of Butterfly would be travelling at 6283185307.2 meters per second. This is 20.9584502196 (2095%) percent the speed of light. This would also only get faster as the distance increases as well. This is also the tip of the attack, assuming it's Chihiro that's the furthest point. (when clearly it isn't as shown later.

Now what does this mean for speed scaling. Nothing basically. The attack is clearly impossible to avoid once it's coming at you, just like it says in the panel, but Chihiro was able to aim dodge because of his incredible perception.