You want as wide a gap as you can get to help burn all the fuel properly, the wider the gap, the longer the spark. Mainly for efficiency when you're out of boost. Most Focus people recommend .045, but that's dependent on a lot of factors. Even .040 is a pretty narrow gap and should be good for 5-7psi. Most would say if you have to run under .040, you should look to upgrade your ignition or wires.

Right now, I'm running .040 at 9psi and it feels fine. I don't push the car hard though, especially now that gas is so expensive.

2000 Rio Red I4 Cougar 200whp & 210wtq at 9psi 254whp & 276wtq with NX 35shot WRX TD04 Turbo, Cut Short Shifter, Strut Tower Bar, 17" ZN Wheels, Roush Springs, Starion Intercooler, NX 35 shot, HKS SS BOV, Full 3" exhaust,StreetFlight Chip