What is the meaning of 后?
For example, 我二十分钟后应该就到了。
I believe 后 means behind? I still don't get how 后 should be placed in the sentence. I thought it made more sense when it was, 我二十分钟应该就到。
How does 后 make the difference?
I think that the use of the character 后 helps to modify the certainty of the statement.
By expressing the exact figures, the person would be trying to convey that the arrival should occur within the exact time (e.g. train arriving at the schedule time at the station).
To create more subtlety in the expression, 后 suggest that the arrival will take place after 20min, so most likely not before or exactly, but most likely after.
However, adding 应该 also suggests uncertainty so they both contribute to modifying the accuracy of the time given.