There should be no tension on the cable when the chain is on the smallest chainring - it should be just barely slack. The low limit adjustment determines where the derailleur cage stops. The most common reason for rubbing as described is low limit mis-adjustment or FD mis-positioning.
Cable tension must be correct for best shifting to middle and/or largest ring. Hi limit adjustment will determine how far away from midline the cage will travel, altho that can be affected by inadequate cable tension.
Take a look at the Park Tool site for step-by-step adjustment advice.