The 1869 issues used the "G" grill which is 9.5x9mm with 12x11-11.5 points. The grills from 1869 on are very easy the 1869 was "G" and the 1870 banknotes were "H" or "I" with "I" being the rarer. Here is a link to a intro to the 1867-1869 grills that might help
http://siegelauctions.com/enc/pdf/1867.pdfEdit: A tip for checking grills is a good scan of the back. With most pic editing software will aloow you the create lines that will grid the grill. This is very usefull in checking the numper of points on faint or preesed out grills,
It is difficult to measure a grill acurately with a ruler alway try to determine the grill type by the number of points.