The longer a line of type is, the more leading is needed so that the eye may more easily move from the end of one line to the beginning of another.
Two typefaces of the same point size may appear to be very different in size if their x-height is different. Generally, the greater the x-height, the more leading a typeface requires.
Inter-word spacing is another consideration: lines of type that have words widely spaced apart become particularly difficult to read with narrow leading.
Finally, the use of the text must be kept in mind. Reference material, intended to be read in small amounts at any one time, can be leaded more closely than a novel or short story. For continuous text, a beginning rule of thumb would provide leading of about 20% of point size, e.g. about 2 points of lead for fonts 8-point to 12-point.
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