How to determine roof pitch (in percent)

The Charente seems to have a very unique way of determining the pitch of the roofs in this area.  Once you know how it is done it is easy but until then it seems beyond understanding. It has taken us three goes, two meetings with government appointed architects and four forum postings but at last we have it figured out - we think!

In it's simplest form the roof pitch has to be 35%. To get this you measure one metre from the roof plate inwards, then at the one metre mark you measure 35cm up. The angle that is formed between the 35cm and the 1 metre is the pitch. The narrower your building the higher the pitch. I have attached a drawing to make it easier to understand.