i think that because sonar or ultrasonic is not that accurate because of the wide angle thing, you might want to do some extra work in program.
if object not detected scan right until object detected else go all the way back left and scan right again
if object detected record angle eg. 90 degrees continue scanning right until object disappears record angle such as 140 degrees.
find the centre of the two recorded degrees and scan to that location (115 degrees) should be the center of the object unless it or you has moved.
while object is "in focus" repeatedly scan left until it disappears, then right until it disappears to find the central location and keep it in focus.
if its for a robot you could give something else to do rather than just chase things; like drive up until bump sensors are activated then use colour sensor to try and probe as much data about the object as you can.
Also if its t chase something, dont forget that if a wall is detected the robot will 'chase' the wall and bump into it such is the same with all static objects so if it detects a wall it will chase into the wall but will only ever detect the wall and not mve onto anything else.
try and get it to only chase moving objects if you can then it has something to follow and chase.
the easy way to do this is a get out clause such as recording the central angle into an array, if the central angle remains the same for a preset number of steps, ignore the object because it is static and look for something else to get.