Boon,
Try this: when uke does a shomen tsuki, you assume chudan kamae, and look at where uke's hands are (here's the key!). You'll notice that the hands "travel" slower and a slightly longer distance relative to the trajectory of the business end. Looking at the hands also increases the depth perception.
So, a slight deflection left or right off your centerline in chudan kame is enough to deflect the thrust of the jo. Sometimes you'll see people do a migi or hidari hasso kamae to deflect the incoming thrust.
In real kumi-jo, you would slide your jo down the shaft of uke's jo and take the fingers of the lead hand out.
In other words, aim for the lead hand when deflecting.