Scale mask from given control point

Question

I have a triangle shaped mask, and want to scale it from one of its points. Now, I am struggling how to do it precisely.

1. There is no coordinates of control point, so can't put 2D cursor there, Shift+S > Cursor to selected also doesn't exist
2. There is no pivot option like Active element from 3D view.

I was thinking about placing 2D cursor at given coordinates and create point here, but even this is impossible, because point is created by mouse and I can't click with such precision.