Rig tension is THE important factor in jib/genoa shape. Most all-purpose cruising jibs/genoas are designed for the forestay to be at approx. 15% wire tension when sailing in 15kts. The forestay tension is controlled by adjusting the backstay tension. The 'basic' tension (for sailing in 15kts.) is 15% on the cap shrouds, forestay & backstay; backstay gets higher tension in higher windstrengths.
Set your rig to ~15% tension, then go sailing in 15kts. and SEE how the rig performs. Here's a link that will explain why you need to correlate sail shape with rig tension: MatchingLuffHollow.gif picture by svAquila - Photobucket
For me, 'too tight' is when the boat is over on its ear when on a beat and the loos gage on the backstay goes 'over' 30% .... above 30% the risk of short rigging life due to 'fatigue' become a vulnerability (most common stainless steel begins to rapidly fatigue when at over 30% max. loading when the 'load cycles' are 'repetitive').