When I installed my window channels I "fit them to the window" first. I clamped a door window into a vice using paint sticks between the widow surface and vice jaws. I measured the distance of the window channel straight section from the retainer in the bottom of the door door up to the "radius " in the door window opening ( tangent point where the straight part ends and the radius begins ) . I then marked the channel section using that length to establish where the bend would start. I marked the window with a Paint line at that same radius and then placed the channel against the window lining up the two marks and then carefully formed the window channel over the window. This allowed me to make that radius bend very accurately.
