Kanna's Kishin Skoukan Spawn Increase

That' is great info. It would coincide with my findings. I had determined it was over 50% but not as high as 75% (rounded down to the nearest whole number).

Could the Y number refer to the skill's hit box though?

No, the hitbox is defined elsewhere. y could reference the rate at which the orb moves, or something like that though. There is also an x = 1.