I don't know where I heard this, I can't find much guidelines online. But is the basic rule for synopsis (for urban fantasy 65k words) one para for each chapter? That seems right. But I'm not sure. Or is that more of an outline... >.<
Think of a synopsis as a sales tool. It deserve the time and effort you put into it. Here is my recipe.
1. Make a chapter list. For each chapter, write two or three sentences summarizing the events in the chapter. Keep a copy of the list because it's often needed for other purposes.
2. Remove the chapter headings 'Chapter 1' etc. and merge it all into one mass.
3. Start editing until it makes sense. Remove material that isn't needed to describe the main plot and any subplots needed to support the main plot. Minimize character descriptions.
4. When you reach three pages, stop. Now, the first time a character is mentioned by name, put the name in ALL CAPS. I also append the age, too, but that's not always necessary.
5. Save a copy to use as your 'long' version, and begin editing the synopsis down to one page. This gets very difficult. You can find examples online which can help.
6. Keep on tuning and fiddling with the synopses. In a sense, they're never finished. Show them to friends and gather comments. Edit some more.
Remember that, when reduced to a synopsis, many stories look the same. So try to leave enough detail in that show your story to be distinct from other stories. Be sure to include the ending. This is not a blurb for the book jacket; this comes later.
Complicated and difficult? Yes indeed, but the better the synopsis, the better your chances of selling.