Can we use present simple to make another future point for future perfect continuous?

Question

Is this sentence correct?

I will have been teaching for 20 years by the time l become 40.

Can I use present simple to make another future point for future perfect continuous?

Answer 1

I think OP's...

1: I will have been teaching for 20 years by then
...is pointlessly complex compared to...
2: I will have taught for 20 years by then

...so I was a bit surprised to see that #1 above puts up a respectable showing on this usage chart...

Note that NGrams has a 5-word limit on search strings, so I did two versions of each tense - one with initial subject I, the other with trailing preposition for - to help focus on the usage in question.

It's interesting to note that when I "drilled down" to look at some actual written instances in context, every single one of the first page of 10 matches for #1 above was from a text intended to teach or test English. Only one of the first 10 instances of #2 was such a "TEFL" text - and it was on the exact same page as appeared among the #1 results anyway! (So it was only really "guilty by association"! :)

In short, this "past continuous in the future" usage is apparently far more popular with people who teach English for a living than people who actually speak English!