For true "hold down" capacity with metal straps typically they should run up the post 2 to 3 feet (it depends on the calculated uplift based on exposure, roof size, and post spacing). Also every hole should be secured with 16 penny nails. But here's the real kicker, the strap should really be embedded in the concrete with a hook in the bottom, preferably looped around a piece of horizontal rebar. The simpson HPAHD22 (see link below) w/ 21-16d nails will give you between 4000 and 5000 pounds of uplift resistance. The strap around the p.t. sill is not considered adequate. That said, it is usually acceptable to most building inspectors to simply toenail the rim joist of a conventionally framed buildign to the sill...so go figure!
http://www.strongtie.com/products/connectors/PA-PAHD-HPAHD.html#gallery
