I thought it wasnt measured diagonally but from the one bottom hole to the other bottom hole and i came up with 80mm, I checked diagonally and i came up with 100mm
i dont get it still though cause the length and height measurement are 80 mm of the space where i mount the fan, but a 100mm fan which is the size of the hole i have has a length and height of 100mm but mine is 80mm :/
edit: what do you think about a intake fan on the side of my case? If you read my post that i made I said something about the fan blowing air under the gfx card which is below my cpu cooler and air might not get to the cooler