I am starting up an algae project, and I am getting a bit confused about some details in calculating density. For example, I take a sample of 250 mL, and concentrate it down to 25 mL with a sand filtration apparatus (factor of 10). I am using a gridded sedgwick-rafter chamber to enumerate the density of algal cells on a cells/mL basis using this formula from Standard Methods 10200F: cells/mL = [(# organisms counted)* (1000 cu. mm.)/ (field area)(field depth)(# fields counted)].
Once I calculate cells/mL, should I DIVIDE that result by my concentration factor (10)? My instinct is to multiply, but wouldn't that be appropriate only if I diluted the sample?