Inevitably, unpublished writers obsess about page counts, word counts, and format. It's hard to understand how your masterpiece in progress stands up, length-wise, to your favorite stories if you don't know how format is done. I think this post by Jessica Faust over at Bookends will help.
Basically, don't obsess. 🙂 I write in TNR 12, though I've tried 14 and I've tried Courier 12. When I want to know how many words I have page-count-wise, I put the whole thing in Courier 12 and do the 250 words per page thing for an estimate. Inevitably, this differs from the actual computer count.
For an explanation why, go see the blog post. It might help. Or it might simply confuse the issue further. 🙂 Just make it readable, and understand whether the publisher you are targeting wants computer count or page count. But don't sweat the small stuff, really. Story is key. Not margins, and not font.
So have you encountered the format nazi? The one who tells you your pages MUST be in Courier 12, or else? I think I was that person once. *sigh* It's what I was told. But, OTOH, I did encounter people writing in TNR 12 and estimating total word count by 250 words per page. Obviously, their count would have been quite a bit short….