No proportional fonts. Right justification messes up margins. 25 best for calculating words per page and book length. This is easy to fix. Get help if you suspect you might need it.
Time is money.
If your book is in bad shape, it may take a copy editor too long to fix to be cost effective for the publisher to buy. The elusive search for verbs Don't say the old man walked precariously down the road. Yes, it says what you want it to, but isn't as strong as possible. Say he hobbled or limped. When writing really pivotal scenes, good verbs will add extra punch.