focalRadius property

The radius of the focal point of gradient, as a fraction of the shortest side of the paint box.

For example, if a radial gradient is painted on a box that is 100.0 pixels wide and 200.0 pixels tall, then a radius of 1.0 will place the 1.0 stop at 100.0 pixels from the focus.

If this value is specified and is greater than 0.0, either focal or center must not resolve to Offset.zero, which would fail to create a valid gradient.