An important point is to be consistent during your calibration. If you keep changing what you do from calibration to calibration, you will have different results during the meditation phase itself. This is to be expected.
If you are too active during calibration, your meditation will be too easy. Basically, you will hear more birds than you should. Don’t keep one or both eyes open during calibration, because this will block alpha waves and produce a too active calibration. So any activity that requires eyes open - such as reading - would not be good for calibration.
If you are too calm during calibration, your meditation will be too difficult. You will hear less birds than you should. Don’t fall into meditation during calibration. Don’t relax too much.
Ideally, you should be in a neutral state while you calibrate. Not too active, not too calm. This would mean a mix of beta and alpha waves. So keep both eyes closed. This will ensure that alpha waves are not blocked. But don’t relax too much. Keep your mind active. Try different things. What I do is count silently from one to ten, and then back from ten to one, in several languages. This keeps my mind busy and active, generating beta waves. It is easy enough to look up on the internet what the numbers one to ten are in different languages. Try this technique, or think up something similar.
Sometimes, though, a calibration technique can become so automatic that you begin to be calm during calibration. When this happens, just switch up with a different technique. Maybe you can think of other lists - the colors (ROYGBIV) for instance. I’ve also performed simple math in my head, like going through the multiplication tables.
Hope this helps,