While the Jamstik app has several custom tunings available from the dropdown in the tunings sub-menu, you may want to customize the tuning on a per-string basis.
You can do this from the settings menu.
Pull up the Jamstik App > Settings > Tuning Settings (at the bottom.)
Defaults (Standard Tuning) are as follows.
E1 = 40 in both the app and on the chart. Please reference the chart for further adjustments.