If you are saying 900mm then it is an alternate method of making a long piece of duct. Stock sheets are 4x8 5x10 and 6x12 feet
so your shop would have to use two 5x 10 sheets to get you a 10 foot section. your alternative would be spiral wrap, or shorter duct sections.
B.E.
The good engineer does not need to memorize every formula; he just needs to know where he can find them when he needs them. Old professor