Sheet.setZoom(numerator, denominator) leaks too much information about how HSSFSheets save zoom level. This zoom function carried over into the XSSFSheet world, where it's inconvenient and clumsy. The spreadsheet API should either use setZoom(int percent) or setZoom(double ratio). A percent (multiply zoom ratio by 100) is already implemented in XSSFSheet, so this seems like the natural pick.
Implemented in r1716048 and r1716053, updated docs in r1716049.