The rule as written states that:
If the attack beats the width or the height of the defense roll it succeeds.

My glancing blows rule would be:
If the attack beats the width and the height it hits, if it only does one or the other it is a glancing blow, and if it does neither it is a miss/block.

A glancing blow lowers the width by 1 for damage only. This rule will lower the lethality of the game some what. To preserve a portion of that lethality make it so that only if the defender wins the width contest, but loses the height contest does the glancing blow rule take effect.

Example:
Jonny Thunderfists punches Sally Sandbag who attempts to dodge.

Round 1
Jonny rolls 6,6,6,4,3,1
Sally rolls  5,5,3,2,1,1
Because Jonny’s attack beats both the width and the height of Sally’s defense she is hit full force, and takes 3 Shock and killing (Jonny has hyper-body).

Round 2
Jonny rolls 8,7,4,2,2,2,1
Sally rolls  10,10,8,5,2,1
Jonny’s attack beat Sally’s width in this case, but her dodge was perfect, if a little ill-timed. In this case Sally takes only 2 Shock and killing as a glancing blow.

Round 3
Jonny rolls 8,8,4,5,3,1
Sally rolls  7,4,4,4,2,1
Jonny’s attack beat Sally’s Height in this case, but her timing was perfect, but her execution off. In this case Sally takes only 2 Shock and killing as a glancing blow.

Round 4
Jonny rolls 8,5,3,3,2,1
Sally rolls  8,8,8,5,5,4
After 3 rounds of getting knocked around Sally finally has Jonny’s number. She both acts in time, and with skill; completely dodging Jonny’s blow.