Yucca filamentosa 'Color Guard'

Does anyone have experience with this one or any type of yucca? 
 
Mine flowered last summer and I cut off the stalk after the flowers were spent.  The plant has been declining over the last year, and it's sent up a pup since.
 
What is the rule of thumb for cutting back the mother plant, if there is one?  I thought it was once the pup is 2/3 the size of its mother, but I think I may be thinking of bromeliads.
 
Thank you for your help!
Bridget
Seattle, WA
 
 
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