You could look at your power duration curve for the last 42 - 90 days and use ~92% of your 8 minute power as a starting point. That should be pretty safe for the first interval unless you are on a bad day. Might be able to pick it up on the last two if on a good day.
This is assuming you’ve semi recently got a decent effort in for around 8 minutes. Should be okay if your fittness has been increasing. If it has been decreasing or you’ve had a dip you would be better looking at the best from only the last 30 days.