The Custom Curve allows you to tailor the falloff exactly to your needs:
In the Lamp subpanel, click the button beneath Falloff: look out for Inverse Square and select Custom Curve from the menu.
Set the Distance: to 47 in the button below Custom Curve.
Scroll down to the Falloff Curve subpanel.
Note that there are three vertical grid lines, and three horizontal grid lines. There is a line that goes diagonally from upper left to lower right. That line is a Bezier Curve. When you click on that line a control point is put there and you can move that control point to change the shape of that line.
Move the mouse over the diagonal line. Near the leftmost vertical grid, press the LMB and drag the curve down so the control point is at the intersection of the center horizontal grid and the leftmost vertical grid.
Press F12.
Move the mouse to the right of the control point you just created and press theLMB over the line. Drag the point down and...