post the real $340 mrr or keep it vague when my employer follows the account Public Metrics

I did the opposite and do not regret it. Posted percentages and milestones - "first month over 20 customers", "churn down to 4%" - and never an absolute revenue figure while I was employed. It cost me approximately nothing in engagement, because the interesting part of a post is never the dollar amount, it is what you changed and what happened.

Also worth remembering the number outlives the moment. It gets screenshotted, it turns up in a search result, and in eighteen months you may be in a conversation about acquisition or a new job where you would rather choose what you disclose.

Post it. $340 MRR is not a resignation letter, it is a hobby that pays for a couple of subscriptions, and everyone senior enough to matter knows the difference. The number that makes a manager nervous is the one that could replace your salary, and that is not this one.

The thing that actually causes trouble is not the figure, it is tone. "Shipped a thing on Sunday, 22 people pay for it, here is what I learned about churn" reads as a curious engineer. "Day job is a prison, escape velocity soon" reads as somebody who has checked out, and that lands very differently in a performance conversation. Same number, completely different meaning.

The written clearance from your manager is the whole ballgame and you are burying it. Make sure it is in an email, not a chat message in a tool the company can turn off the day you leave. Forward it to a personal address today. I have watched somebody lose that thread in a migration and then have a very unpleasant six weeks reconstructing what was agreed.

Small practical thing: whatever you decide, be consistent. The people who get burned are the ones who post the number in a good month and go vague in a flat one. That pattern is obvious from outside and it makes everything else you post look like marketing.

Nobody at your job cares about your side project nearly as much as you think. I posted mine for two years, my director occasionally left a thumbs up, and it never came up in a single review. The anxiety is real, the consequences mostly are not.

Depends entirely on the employer. If you are anywhere with a compliance team and a policy about outside business activity, the exposure is not "my manager saw the number", it is "someone in a different building found a public revenue claim and now there is a process". Check whether you have a declared-interests form before deciding this on vibes.