***** NOT A TEACHER *****
I am NOT answering your question.
I just thought that you'd like some information that I found.
1. This is what one book says (my emphases):
" 'Should' is usually substituted for 'must' to soften a demand or requirement but as with 'must,' there is no choice involved."
-- Andrea DeCapua, GRAMMAR FOR TEACHERS (Google "books").
2. "How early should I arrive at the test center ...? Arrive ... 20 to 30 minutes before the time designated on your admission ticket."
-- Jerry Bobrow, LAW SCHOOL ADMISSION TEST (Google "books").
Student or Learner