According to Pam McCutcheon, when you are discussing your manuscript with your publisher your word count should be based on 250 words a page (1 inch margins, 25 lines per page, 12 point non-proportional font) and not the actual word count. This is because some parts of your manuscript may have fewer words per line than in general (ie: dialogue) but take up more space. The publisher is interested in how much space your manuscript will occupy.

Pam McCutcheon does not mention this, but long words take up more space than short words, and that is not taken into account in the 250 words per page formula.