Well if I’m not mistaken, L&R Pre Fader is usually the best selection for recording a board mix. The reason for that is if you change the output level of your PA a fair bit, it won’t affect your recording as the signal is being taken before the L&R Fader control. If you want to be able to fade in or out your mix going to your iPad. Then select post L&R. Just remember that when you move your L&R Fader it also lowers then signal going to the output jacks to your PA system.
NB. If you choose to use PreFader, it is like you have set your L&R Fader to unity (0dB) on your mixer and it is locked in that position just for the iPad and not for the L&R outputs on the unit.
Hope that helps.
Cheers
Al T