I humbly suggest that some of my friends here brush up on their Maths skills.
Marianne, I don't know who said that your comments were inappropriate, but I'm going to add my opinion: I do not have the time to do this BOM (or any of the previous ones), but I have read this thread with great interest. Please, ignore anything negative and focus on those of us who enjoy a good discussion.
I'm all for instructions for the use of 'gadgets' are includes whenever suitable, as long as methods that don't need them are also given. Not all quilters have the money to buy lots of expensive 'stuff'.
I feel the same about speciality rulers.