[Moon] septum feed question

[peter blair]

 Referring to Zdenek's ( OK1DFC) septum calculation file  which  you will find at http://www.ok1dfc.com/eme/technic/septum/calc.xls  can anyone tell me what is the significance or importance of the dimension K, the distance from the end of the last tooth of the septum to the open end of the guide.
  Im not questioning Zdeneks excellent work but I cant understand  why this should be a specified or critical  parameter.
If it is a critical parameter in the original ( square ) design is it also critical in a circular guide?
  Im asking the question widely because I guess by now quite a few people will have built these  and there should be some experience out there Thanks, 73 Peter G3LTF 
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