Mastodon: Show character limit for cross posting, such as to Twitter

[sparr]

Pitch

When composing a post I see a countdown of how many characters I have left before hitting the Mastodon post length limit. Since my toots are cross posted to Twitter, I'd like to also see a countdown to Twitter's limit.

Motivation

This would help avoid truncated cross posts.

[Kjwon15]

Twitter doesn't count 1 character as 1 letter.
Korean character are counted as 2, space/latin character are counted as 1.

And I think this doesn't needed. Crossposter can split your post into several tweets.

[clovfr]

I don't think it's relevant for Mastodon, because Mastodon is not a Twitter client.
You must do that with UserScripts.