Could anybody correct my sentences?

Do you want to eat it?
No, I have just eaten my lunch at two.

Please help me confirm that whether my ideas of present perfect tense is right.

These are the conclusions that I made after reading all those posts.
Present Perfect can appear in a sentence with clear time indication(at two) like the sentence above.

I don't know whether simple past tense or Present perfect tense is more appropriate in the sentence above.
Please give me two examples that are more appropriate to use present perfect tense but with clear time phrase if there are.

Many Thanks in Advance
Hi there

You can say:

"I've just had my lunch"

WITH NO time reference.... this way the situation is OPEN and as such it has a present effect. The point is you are giving a REASON why you don't want to eat NOW, so you use the present perfect.

But you can't use the present perfect with a CLOSED time reference (at two) as this distances the action (having lunch) from the present. The time (at two) is CLOSED and FINISHED.

The sentence "I have just had my lunch at two" also doesn't agree with itself. You can't "have just had" and also "had at two". It's one or the other!

Hope this helps
It was better to say "No, I have just taken my lunch at two"
Even better is: "No, I have just had my lunch at two"

Present perfect is used for events that have just finished, or past events that have some influence on present:

"I have taken my lunch." Implies that I am not hungry anymore.

Simple past is used for past events that are, in a sense, disjoint and do no affect the present:
"There lived a great king in Arabia." Since the king has no use in present time.

There are many more uses, but I'm trying to keep it simple.

Hope it helped.
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 anonymous's reply was promoted to an answer.
Vctory Ong
Present Perfect can appear in a sentence with clear time indication(at two) like the sentence above.

No, that's exactly the opposite.

With a clear time indication, do you not use the present perfect.

"Thanks, I've already eaten," but "I ate at 2 pm."

Just a note about Horizon's comments: In the US, we don't use "take" for lunch. We use "have" or "eat." We use "take" to talk about the break in the working hours to do something - so you might "take lunch" at 2 to mean that's when you have your lunch break, but it's specific to referring to the interruption in your work schedule, not the meal.
In addition to G.G.'s comments, "take lunch" has a specific meaning but can't mean "have just eaten". If a business employees 3 sales clerks and each must take turn for lunch, X may say to other 2 coworkers "would it be ok if I "take my lunch at 12:30?". In this context, it means "have this time" for lunch.

Did you take your lunch? This means " DId you get your turn to lunch?"

Yes, I took my lunch half hour ago. This can imply "you just ate".
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