As far as cable tension, the cable should be pulled taut with the derailleur in the low stop position, nothing more. Fine adjustments can then be made with the barrel adjuster if necessary. It's kind of sounding like your front derailleur doesn't have the range of movement necessary to shift to the outer ring. Is it a derailleur originally intended for a double crank and you're using it with a triple? Sometimes this will work, sometimes not-