The cap is designed for use with differently sized rear headliners, which are included in the cap's accessory kit. The rear headliner should be replaced with a thicker headliner from the accessory kit if you feel you need a more snug fit. The cap should not feel tight on your head, but also should not shift around during treatment. As long as the sensor detects that the cap is on your head, and it is neither uncomfortably tight nor shifting during treatment, that is a good fit. Once connected to the REVIAN app, you will be notified via an on-screen alert if the sensor isn't making contact.