Welcome to AW. Uncle Jim had a bit to say about why publishers keep using the 250 words per page method. It tells them how much paper they will need when printing the book This tells them the printing cost. Comparing the two methods, you will always get more words using the 250/page method, because dialog normally doesn't fill the whole line across the page.
In order to be accurate when using that method, you should use a non-proportional font such as Courier. The size should be 12 pt. (Some people say 10 pt will also work.) Margins must be 1 inch all around and you MUST have 25 lines of print on the page. Occasionally, you will end up with a page with 24 lines which can't be helped. If you don't have 25 lines of printed material on most of your pages, cheat a little with the top and bottom margins. Make them .9 inch.
May I recommend that you read Uncle Jim's thread. It will answer questions you didn't even know you had.