If you have not installed any blocking at the bottom this point will be prone to failure with any flex in the wall.
Have you seen how we use Kerdi Fix and Kerdi Band to tie in a tub's flange to the wall substrate? This is your best approach and will help add a little strength to the job if the blocking was missed.
I would never use Kerdi Board unless the wall stud spacing was under 10-12" on centres. To much flex.
I have a good review of Kerdi Board here: http://www.terrylove.com/forums/show...22-Kerdi-board
The best Kerdi Board is home made. You take concrete board. Install it. Tape your seams. Then apply Kerdi over it. That's what I call Kerdi Board. The flimsy foam boards that use cardboard for strength are crap and best avoided in any install.