open boundary condition in CST

hi
Do you know what is the optimum distance from air box in CST?
what is the best answer for adding space to open boundary condition?
when I use lambda/4 and lambda/2 in central frequency the answers are different.
thanks

The default in CST is lambda/6 for the center frequency and for me it works good. But in guides for HFSS i've read always lambda/4 and it works too.

Change the distance unless s-parameters don't change anymore.