When you say something out of conjecture or by guess

Question

Let's suppose someone asks you about their age. E.g.

(A) What do you think? How old I am?

(B) You seem to be maximum of 35.

(A) Wow! Exactly. How did you say that?

(B) I said it...........

• 1. conjecturally
• 2. out of conjecture
• 3. by guess

To me, no one of the above choices sound natural. First because the my spellchecker marks the adverb "conjecturally" as an incorrect one, then based on Ngram shows that the self-made term "by guess" is used very rarely.

So please let me know what is the most natural way to indicate that you say something out of a guess / conjecture?

Answer 1

At first, there are some problems with your dialog.

(A) What do you think? How old am I?

Inversion since it is a question.

(B) You seem to be maximum of 35.

It is unlikely someone would use "maximum" this way, as it indicates any number up to 35. This conflicts with the next phrase.
So I would just say "You seem to be 35" or "I think you are 35."
If you do use maximum it needs an article as in "a maximum of".

(A) Wow! Exactly. How did you say that?

This should be "Why do/did you say that?". Either past or present tense is OK since it was just said a moment ago.

(B) I said it because ...........

The best way to deal with this is to use because with a phrase explaining how you decided on the age. If you actually guessed, then just say "Lucky guess."