I never really bother with the backstay while tuning uppers and lowers. The backstay adjuster block/triangular fitting at max ease should have just enough tension to keep the backstay from flopping around. In light air with the mainsheet on it, the backstay should be loose enough sot that it is easy for the main to pass through during tacks and only a quick flick is need during the gybe.
When you tension the backstay adjuster, you should be able to pull it on until blocks reach the pushpit, at which point it should be very tight. If looks to be breezy (15+) tighten the backstay turnbuckles, since the purchase works more efficiently at the top of the split than as it nears the bottom.
You may need to trim a couple inches out of the primary wire if after you trim the backstay adjuster to the pushpit and it is still not tight or if you have to max the turnbuckles to take out the slop.
Sighting up the forestay with the genoa set you will see headstay sag to leeward and aft consistent with the way the sail is cut. If it is too loose don't be afraid to take a few turns at a time until it looks right. Unlike a masthead rig, the backstay on a fractional does more to open up the leach of the main than tighten the headstay. Start pulling on tension only when you need to depower.
