Hi,
Macros are things that users create to do repetitive things in Word.
Word Macros and Visual Basic for Applications (VBA) FAQ
There are a lot of macros that other users have written that you can find online, but none for this as far as I know.
You might want to look into teleprompter programs instead of Word.
Word does have built-in commands which often function much like macros, but there is no such Word command and I think a macro like this, which essentially is monitoring every keystroke, would interfere in the functioning of Word. I possibly could write one but it would take me at least a day (I'm not that good) and I frankly do not think the benefit it would give is worth a day of my life.
I think the method you are using is the best one. It gives you the roughly 150 words per page and the idea of 150 words per minute is also a rough calculation.
You have tools built into Word that can help you. You can have the number of words in your document displayed in your Status Bar. It is relatively easy to divide by 150 and figure the number of minutes. You can press Cmd+Enter after every 150 words if you want.
I would be happy to write a macro for you that will add a page break after every 150 words in a completed document.
See also:
Word Doesn't Know What a Page Is