does your pentax have TTL (through the lens flash metering) flash capability. If so buy a dedicated flash (one designed specifically for the camera). Then you can can Ian's advise by simply dialing in a flash exposure compensation of -1 or -2 for fill flash. TTL will work with any fstop you choose. You can dial in compensation in 1/3 stop increments. For fill flash in bright sunlight, I think -1.7 stops is an ideal starting point.
Take care,