A question of licence under usage.

Phil C directed me in another thread to have a look at the post by JBenoit http://communities.bentley.com/products/licensing/f/275813/p/94262/276243.aspx#276243 

regarding under usage. The post states:

would like to address the comments about underuse of licenses. Please remember that license pooling remains a benefit with SELECT Open Access. License pooling dramatically improves your software ROI by allowing licenses to be shared and streamlining asset management and allocation of project work. The SELECT Open Access subscription enables subscribers to share licenses across all sites within a given country.
…which doesn’t answer the question! In fact it does anything but answer the question.
I have spoken to our Account Manager about this but  I was palmed of to someone else who also didn’t answer the question of under usage.
All not very helpful.
If out of 90 days I over use 12, then 78 are underused, At what point does the calculation work out that we should be billed? Is it 46 over and 44 under, or what?
Please could Bentley can confirm how under usage is calculated?