Here are my opinions. Two different
should's are possible:
the should of expectation should-exp
the should of advice should-adv
should-adv is much more common than should-exp.
should-exp
expresses an evaluation of the situation with regard to what is
possible, probable, plausible, or necessary logically.
should-adv expresses an evaluation of the situation with regard to the propriety of the behavior of the people involved.
Because
of the different ways a situation can be viewed, depending on the actual content of the sentence, those situations
containing more 'agency' (subjects of the sentence referring to people,
not things) are more likely to have a reading of should as should-adv. In fact, the interpretation of should as should-exp can only come through in a convincing way when the sentence is interpreted in a way that leaves questions of agency aside.
She should-exp be at John's. = I expect that she is at John's. (giving an opinion about her location only)
She should-adv be at John's. = It is advisable for her to be at John's. (giving an opinion about her duty, her behavior)
She shouldn't-exp be at John's. = ???I expect that she is not at John's. (Rare, I suspect.)
She shouldn't-adv be at John's = It is not advisable for her to be at John's.
She should-exp have been at John's. = ???I expect that she was at John's. (Rare, I suspect.)
She should-adv have been at John's. = It was advisable for
her to be at John's (although she was not there). = She was supposed to
be at John's (although she was not there).
She shouldn't-exp have been at John's. = ???I expect that she was not at John's. (Rare, I suspect.)
She
shouldn't-adv have been at John's. = It was not advisable for her
to be at John's (although she was there). = She was not
supposed to be at John's (although she was there).
Note that as the negative and the auxiliary have are added, it becomes increasingly difficult to assign the meaning should-exp to the should in the sentence.
__________
She should-adv leave for America. = It is advisable for her to leave for America.
She shouldn't-adv leave for America. = It is not advisable for her to leave for America.
She should-adv have left for America. = It was advisable for her to leave for America, though she failed to do so.
She shoudn't-adv have left for America. = It was not advisable for her to leave for America, though she did anyway.
________
There
is so much 'agency' in the idea of someone leaving for America that it
is very difficult to see how these sentences could be construed as
containing should-exp. Changes must be made to the sentences in order to cue the reader to the possibility of a reading with should-exp. Note the use of the progressive tense to help cue the reader toward should-exp.
She should-exp be leaving for America soon. = I expect that she will leave for America soon.
She shouldn't-exp be leaving for America for some time. = I don't expect her to leave for America for some time.
She should-exp have left for America by now. = I expect she has already left for America (by now).
She shoudn't-exp have left for America yet. = ???I don't expect that she has left for America yet.
The last two might more easily have been stated with will or would, the negative probably being less used than the affirmative:
She will/would probably have left for America by now. = I expect she has already left for America (by now).
She won't/wouldn't have left for America yet. = I don't expect that she has left for America yet.
_________
This task should only take three minutes to do. (should-exp because there is no agency.)
He's
only going to mail a letter. He shouldn't be long.
(should-exp because there is no agency when we merely estimate the time
an action will take. There is nothing here to suggest that we
would be evaluating the propriety, appropriateness, or correctness of
his behavior.)
________
I hope this helps, even though it is a bit complex.
CJ