Unclenick, I've got this solved.
The term that should be used for this measurement is 'Casefit'.
As in, 'for optimum accuracy you should make sure your Casefit is two thousands shorter than the Headspace'
And with the implied nouns added:
'for optimum accuracy you should make sure the Casefit of your case is two thousands shorter than the Headspace of your chamber'
There, world peace
