20% whitespace in text body, margins should be around 40% of the width, for English textbooks, preferably including a column down the left side of the page, or split over both sides, or down the right side as a last resort if sufficient room on the left is not available).
20% empty space in text body, margins should be around 40% of the width, preferably for English textbooks, including a column down the left side of the page, or split over both sides, or down the right side as a last resort if sufficient room on the left is not available.