Headers display incorrectly on zoom settings higher than 1.54

[tobiasnteireho]

setting WKHTMLTOPDF_ZOOM higher than 1.54 makes the headers display incorrectly. I think 1.54 is a good enough compromise for now as this will be nearly unnoticeable at smallish/normal font sizes and is short by less than 10px on the 72px cal font but this needs to be fixed before the next DAT release. Correct setting is likely closer to 1.6 but looks like it has changed again #103