Flex has about twice the static pressure drop as round metal duct if pulled taught. Any slack adds extra drop, as do excessive bends which are all too convenient when using flex.
Over short distances, static pressure drop is largely irrelevant. The fan manufacturer should have a table stating what the equivilent head loss is, often listed in feet for a certain diameter of duct. Add 15' for every 90 degree bend in the run, and then double it for flex duct.
