@johngod Thank you so much, you're right Screen.pixelDensity would be the complete solution if it was in qml,
sifting trough the qt source i was able that it was derived from:

double pixelpermm = QGuiApplication::primaryScreen()->physicalDotsPerInch() / 25.4 //physicalDotsPerInchX() & physicalDotsPerInchY() also exist

and it seems to be correct on both my iPhone & iPad quite wel (better than all other ways I found so far)

@Lorenz thanks, but for the time being I want to stick to Qt as it is a paid sdk as soon as you go commercial