I used a simple distance calculation:
distance(Mouse.X,Mouse.Y,Ball.X,Ball.Y)
This will set the force directly to the distance from where the mouse was clicked (Mouse.X and Mouse.Y) to where the ball currently is.
You can adjust this force by multiplying it afterwards if you want.
Half the force: distance(Mouse.X,Mouse.Y,Ball.X,Ball.Y) * 0.5
Double force: distance(Mouse.X,Mouse.Y,Ball.X,Ball.Y) * 2