Word count for English A level coursework - how do I do this?

my english a level coursework has to be less than 2500 words long, and on the microsoft word count it comes up as 2870, but it counts white spaces as words so i know this isn't accurate. someone told me to use courier 12 font and then the word count is 250 per page, but this can't be accurate either? as words vary in length. how accurate does my coursework word count need to be (i need to write it on the bottom of my work before submitting it), and which is the best way to do it? thanks x