I can find in the manual advice on how to reset the focus point after using the Touch Focus in the other cases than Flexible Spot. However, I have not been able to find an easy way to reset the focus point back to center when using Flexible Spot as the Touch Focus mode (Focus Area).
Very often I accidentally touch the screen and the focus point can move from center. Now I do it by going through
1) Press Fn
2) Select Focus Area
3) Move the focus area back to center manually
This is a slow way