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Problem with QUEST or quest (what's the difference?)

Win10:
Builder 1.85.3
Standard Standalone? (y)

Hi all,
I’m trying to convert a simple staircase I have been using into a QUEST staircase. My first question is what is the difference between the two options in the loop in builder (QUEST vs quest)?.

 My next question is more specific.  I have a 100 step continuum (between two speech sounds) and I'm starting people off at the 1/2-way point (50).  So I set the startVal at 50 and the minVal at 1 and the maxVal at 100.  I also set the range at 99 (difference between maxVal and minVal) and the StartValSd at 15.  Since I want to find the boundary between the two sounds, I set the pThreshold to 0.5.

  I immediately get an error saying "psychometric function range [0.5 0.99] omits 0.50 threshold"

 I assume I'm misunderstanding the units involved, but I don't know which parameters I'm misinterpreting.
 Thanks!
 Mark