I just released my ‘Count Characters for Micro.blog Timeline’ shortcut version 1.0:

This shortcut converts Markdown text to Rich Text and then counts the number of characters. If it is over the character limit to be displayed in full on the Micro.blog Timeline, it allows you to edit the text to be within the limit to avoid truncation. If the edited text still has too many characters, it will continue to rerun this shortcut until it is within the limit.

Get it here from the HeyDingus Shortcuts Library.