Don’t worry, use the 5mm rubber shim in conjunction with cross spacers.
Detailing a whole range of products https://www.wallbarn.com/projects-case-studies/
Our factory has just been awarded the ISO 14001 certification for its effective environmental management system. Bituminous Protecto-board is manufactured in accordance with the ISO 14001 standard.
See here for more details on the product – https://www.wallbarn.com/products/roof-structural-protection/protecto-board-bituminous-membranes/