Witchipedia

You can redo any spell as many times as you like.

I'm curious to see if your spell has helped your hair grow. If so do you mind sharing?

I highly doubt chanting centimeters instead of inches "messed up" your spell because (in my opinion) willing your hair to grow is highly unlikely to cause physical results. I suggest, for physical results, using plant and animal spirit magic instead. For example aloevera gel, eggs, ginger, and flax seed can all stimulate hair growth (among many others), and they can all easily be made into a hair mask you can apply to your hair. Also there are certain foods you can eat that can stimulate hair growth. I know these methods might not have the mystical appeal that other forms of magic have, but they are magic all the same and have been proven to work.
Also, just like plants, hair needs to be stripped of the "dead" parts to give way to new growth. It may seem contradictory to your goal but trimming the dead ends and damaged parts of your hair will increase its growth rate. Everything functions by the rule "as above to below, as within so without" and vice versa. It's a known fact that by trimming the spent pieces of plants it can give way and make it easier for that plant to put its energy into producing new growth. Same goes for many other things, including our hair. Good luck.