That's one of my all-time favourite quotes, but it turns out Maugham may be worse at maths than I am, because The Guardian have asked lots of fiction writers for their 10 Rules for Writing Fiction and they've assembled rather a lot of them.
Contributors include Neil Gaiman, Roddy Doyle and Margaret Atwood, among many others, and some of the rules are pretty funny so it's worth reading.
Here are a few of my favourites. I especially love Geoff Dyer's advice about Nabokov.
1 Take a pencil to write with on aeroplanes. Pens leak. But if the pencil breaks, you can't sharpen it on the plane, because you can't take knives with you. Therefore: take two pencils.
2 If both pencils break, you can do a rough sharpening job with a nail file of the metal or glass type.
3 Take something to write on. Paper is good. In a pinch, pieces of wood or your arm will do.
Do not place a photograph of your favourite author on your desk, especially if the author is one of the famous ones who committed suicide.
Do not search amazon.co.uk for the book you haven't written yet.
Do spend a few minutes a day working on the cover biog – "He divides his time between Kabul and Tierra del Fuego." But then get back to work.
Keep a diary. The biggest regret of my writing life is that I have never kept a journal or a diary.
Have regrets. They are fuel. On the page they flare into desire.
Marry somebody you love and who thinks you being a writer's a good idea.
The first 12 years are the worst.
The way to write a book is to actually write a book. A pen is useful, typing is also good. Keep putting words on the page.