When is my next 90-day report due after returning to Thailand on October 7th?

Here's an easy way to remember this: the 90-day report is due when (and only when) you have stayed in Thailand for 90 consecutive days. The counter (re)sets to 1 when you enter Thailand. Each day you stay in Thailand the counter increments by one day.

If you are in Thailand on the 90th day the report is due and can't be avoided by leaving Thailand. You do, however, have 7 days after the due date to file your report in person. If you leave before the 90th day no report is due and the counter will reset to 1 when you re-enter Thailand. You can file in person up to 15 days before your report is due. You can file by mail before the report is due if your immigration office allows. You can do the report online but only during the window from 15-7 days before the due date (90th day).

You don’t have to go exactly on that date. I believe it can be 15 days before and 7 days after the 90 days.

90 days from return