Most important is to stick to the same test and keep the testing workflow as consistent as possible between tests. Also, realize that every person is different in terms of how the average power in these tests translates to an FTP value. The FTP result might need to be scaled up or down before you can use it to determine interval intensities.

For example if 90%FTP efforts for 10 or 15 minutes are a walk in the park or if it feels like an all-out effort, you might need to adjust the FTP value. Next time, if you use the same test you can then apply the same personal adjustment to get the updated interval intensities.