Set the bend by the bulge you can measure
Radius is impossible to drag: on an almost straight wall it runs into hundreds of metres and jumps with every twitch. So the bend is the bulge, how far the middle of the wall moves from the straight line between corners. On a real bay window, stretch a string between the corners, measure to the wall in the middle and type the number in. Plus bends outward, minus inward, and the radius is shown for you.
Area, elevations and estimate follow the arc
A 3 m bay with a 60 cm bulge adds about 1.2 m2 of floor. Wall length is measured along the arc, so paint, wallpaper, wall elevations and the estimate account for it.
A shared wall bends in both rooms
Bend a partition in one room and it bends in the neighbouring room too, with the opposite sign, so no lens-shaped gap opens between them. Whatever the living room gains, the bedroom loses.
Doors and windows sit on the curve
Openings are placed on the arc itself and measured along it, so a window in a bay stays in the wall at any bend. Auto-furnishing treats curves conservatively, so a sofa never ends up inside a bent wall.