"That should do it / That ought to do it", do you say it before the last thing is done or after?

Question

Longman Dictionary says:

that should do it also that ought to do it (spoken):
used to say that you will have finished doing something if you just do one more thing:
I've just got to prepare the dessert and that should do it.

So you should say it before the last part of the job is done. But many times I've encountered this idiomatic expression that it was said after the job was finished off. I did a search in Google Books:

• The airplane kept climbing. That should be high enough, I thought. The plane continued to climb. That should do it. But no, we continued ascending. C'mon, we're really high enough already! Hey!

Furthermore, this usage is suggested by all the answers to this question. For example:

If I bake a cake, let it cool, put icing on it and add some decorative sprinkles, after the final sprinkles are added I might step back, examine my cake and say, "That ought to do it!" Which signifies both to myself and anyone present that the cake has now been completed to my satisfaction and is now ready to be eaten.

So there should be no doubt that "that should do it" could mean "that should have done it". I have two question:

1- Is it used before the last part at all? (as Longman Dictionary claims)
2- Why is it in the present tense? Why isn't it like : "that should have done it"?

Answer 1

In my view,

1. Yes, the Longman Dictionary usage seems right and in my experience, I have seen this phrase used like that. Most often, it is said just after doing the last part. Like once, I forgot to tip at restaurant, and waitress was mentioning that it is customary to tip, and one of my collegues added few dollars extra and said "That should do it"

2. "that should have done it" seems more apt when, even after doing the last part, something was not accomplished. Something like "I posted the mail, and that should have done it. However, the mailbox was inoperative and it never got sent"