When the Select Intervals button is tapped, a new screen is presented where you can decide which intervals should be used to control the amount of stretch in the bass and the treble. The normal default for these intervals is the six three octaves for the bass and the four one double octaves for the treble. This means that the stretch in the bass is adjusted to make six three octaves as pure as possible in the low bass, and four one double octaves as pure as possible in the highest treble. Between the highest treble and the lowest bass, the stretch gradually changes so that it is a blend of the two criteria for notes in between. Other intervals produce more or less stretch. For example, if you select four two octaves for the treble, you will get slightly more stretch in the treble. And if you select eight four octaves for the bass, you will get slightly more stretch in the bass.
At the top of the interval selection screen are two spinners. The left spinner selects the bass interval and the right spinner selects the treble interval. After setting the intervals the way you want them, you can go back to the main tuning screen by tapping the Done navigation button in the upper left corner of the screen.
Any change you make this way will only affect the current tuning file. It will not affect any other tuning files, past or future. But suppose you want the new interval selection to be your personal default for all new tuning files. Then you can tap the Make Default button in the lower right portion of the screen before returning to the main tuning screen. Then every time you make a new tuning, those interval settings will be in effect for you automatically.
The two-fingered Zee-swipe escape gesture can be used to exit from this screen.