How do I calculate my 90-day report due date for a Non-O visa in Thailand if I re-enter the country?

90-day report for non-o married to Thai wife, posting this as it was new to me but may be already common knowledge:

I was on a non-o married to Thai spouse extension of stay which was due to expire mid July. I left Thailand to go renew my passport (U.K.) and re-entered Thailand on 6 July and promptly proceeded to renew my non-o.

Soon after I received an email from immigration saying I had to do my 90-day report by 28 August and after attempting this online it was rejected so I went to CW to file in person.

At CW I was told by the very nice IO that I should have ignored the email and counted 90-days from my most recent entry stamp. That would be the date of my next due 90-day report.

Again this was news to me but may be useful to someone else in a similar situation.