Hello Petr!
It's a fascinating APP you have developed and the possibilities are endless!
My first impression is: the digital pressure sensor of the phone (in my case a second-hand Samsung S3 in good condition) is remarcably accurate, and I suspect it is comparable to traditional IFR-certified altimeters, which is as far as I remember +/- 60 feet accuracy.
Of course, what you get is cabin pressure, but this is good enough for most of us.
My suggestion for a possible improvement to one of the next editions of Flyisfun is this: To join the 2 pressure-calibration procedures in one single page, namely in the Flying Settings/QNH page, by moving App Settings/Preferences/Sensor Settings/Pressure Sensor Calibrating-function to the Flying Settings/QNH, thereby saving around 6 tappings in all the submenus.
kind regards
findan