I'm curious about this setup as well. Apparently, the center sack will fill first but since the thru hull vent is higher relative to the IBS, the IBS should then begin filling from the third leg on the 'T' prior to either bag venting over board. Something to do with physics and fluid dynamics... I'm about to undergo the same install and I received the same instructions from the wakemakers gang. I'm not sure what the pros/cons would be vs attaching the IBS fill from the center sack vent??...