Learn English – Use of the future tense after the phrase “I’ll let you know when…”

I think it's often expressed with the word next, in place of the now, particularly when it's used to form an exasperated exclamation:

Next you will tell me that Rome isn't even in Italy!

I did a Google books search which revealed several contemporary examples. When I performed a similar search using "now" in place of "next", there were several results returned, but most of them seemed to be in an interrogation setting, not the ironic utterances you point to. Here are a small handful of samples:

"Next you will tell me they are sending aeroplanes to the stars, just like to London."

"Don't tell me fibs" — the teacher sounded annoyed — "why next you will tell me that you speak fluent French."

"Now you will tell me what you really are. You will tell me why you have come here with Lieutenant Halfhyde."

"Now you will tell me what is happening, unless you wish to have me tied to my horse's back and carry me screaming aloud all the way to Cornwall!"

I'm not claiming that the word "now" couldn't be used in this way, but I think "next" might be a better choice.

There's also the idiomatic "Now you tell me," which is sometimes said when someone reveals some information too late:

"Oh! I forgot to tell you! We'll need to go pick up Becky, too!"
"Now you tell me! We'll never get there on time."

SHORT ANSWER:
Only your first sentence is idiomatic English.

LONG ANSWER:
First, will is employed in when clauses only if when is employed as a relative adverb, not if when is employed as a subordinating conjunction:

 You will arrive in Milan at 4 pm, when you will be met by our representative.
^✲When you will arrive in Milan you will be met by our representative.

Second, the ‘sequence of tenses’ rules for sentences with [when] ... [then] and [if] ...[then] constructions are very similar, so I’ll treat them together.

NOTES: I bracket [when], [if] and [then] because these words may only be implied and not actually appear in a sentence. Also, I have changed the words in your example to permit what I hope is a more transparent explanation.

The rules vary depending on whether the sentence expresses habitual practice or, as in your example, a single contingency.

1. If the sentence expresses a repeated or habitual practice, both clauses will have the same tense: either simple present or simple past. In these cases, [when] or [if] bears the sense whenever.

   [Our practice is: ] Whenever we see this, we do that.
   [Our practice was:] We did that whenever we saw this.

2. When the sentence deals with a single occasion, English employs a sequential structure, in which the [then] clause is depicted as ‘following’ the [when] or [if] clause. In practice this means that

   • In sentences with present reference, a simple present or present perfect in the [when/if] clause is paired with a will + unmarked infinitive construction in the [then] clause.

     NOTE: 1) The order of the clauses in the sentence is irrelevant, and 2) I speak of a ‘construction with will’ rather than of a ‘future tense’ because the will signifies consequence quite as much as it does futurity.

     [We have decided:] If we see this, we will do that.
     [We have decided:] We will do that when we have seen this.

   • Sentences with past reference are ‘backshifted’: a simple past or past perfect in the [when/if] clause is paired with a would + unmarked infinitive construction in the [then] clause.

     [We decided yesterday:] If we saw this, we would do that.
     [We decided yesterday:] We would do that when we had seen this.