1p hypothesis test

Dear Six sigma professionals,

i did 1 p test to confirm the subject matters statement that 25% of the received cases are not quick killed due to Handoff.
Q1 Can i do 1 p test to confirm this
Q2 one of my friends is asking from where the Subject matter expert got this 25% figure. why not 20% or 30 %.
Q3 how to select the 25% or any other value, is it a guess by sme or should it be from well know standard value

I did 1 p test as per my understanding.
I want to know others view.
will appreciate your responses
Thanks
Navin Rohilla