Hi Grzegorz @Kovalsky ,
I don’t know what you mean exacctly, but Midpoint Snapping by holding SHIFT works on any Edge. Just hold SHIFT at the right time, when cutting the Edge you want. So you can start the cut anywhere on an Edge and then cut towards a second Edge (the first Edge cut is now fixed) and hold SHIFT and Cut the second Edge (click to confirm and then Enter).
Admittedly, the Knife Tool does need a bit of practice!
