Our own experience has shown that pathfinder's mechanic for calculating movement speed is cumbersome at best, and often ignored by players. As such, we have decided to use an alternate method, and set a players movement speed (in feet) will be equal to his 20 plus his strength, minus his armour's penalty rating, rounded down to the next lowest increment of 5 feet.
Example: Private Jimmy has a strength of 14. His speed would be 34, rounded down to 30 feet per round. If his strength were 16, it would be rounded down to 35 feet per round. If Jimmy were to wear Combat Armour (which has a penalty of -5) his speed would be reduced to 25 feet per round (or 30 feet if his strength was 16).