If you are reducing the size in patchwork, just using percentages should be the easies route. For half-size, multiply sizes by .5, 75% multiply by .75
stick with nice round numbers and make up shortfalls with wider borders.
Most patterns break down into sub-units of 3-4-5-6- across and tall. Resize each proportionately. If you are using a classic pattern, chances are good that other sizes have already been published. What block are you talking about?