Here is a spreadsheet for calculating rigging diameters. I have included an Excel 2010 file and a PDF printout.   Enter a value for the mast diameter in the green cell and all rigging sizes will be calculated. I label the mast diameter as "Model mast diameter" but you can also enter the real ship mast diameter. Use English (inches) or metric (millimeters) and the resulting calculations will be in the same units. The mast diameter 0.320 inches is for the 1:48 scale topsail schooner mode