Eagle Users
As you know Eagle is grid oriented. Most electronic parts are based on inch measurements and fractions thereof. PCB mounted transformers are mostly based on a metric scale, e.g. pin distance 5mm (close to 5.08mm=1/5inch) If the pin diameter is 0.8mm and you select a drill size of 0.9mm you can compensate for that inaccuracy. The problem starts to add inaccuracies at bigger distances. Choose the nearest value converting mm to inches and place the pad there. To make the outline of a device switch the grid from inch to mm display and select a suitable grid size, e.g. 1/2mm. Draw your device outline at a new origin. Draw two diagonal lines (grid inches) using layer 21 and zero line width from the outer pads of the device. Next put the zero reference point exactly on the center. Delete the lines you have drawn, hide the pads, group the dimension outline, based on mm and move it compelely to match the center point you have selected, with a center cross you have made before. The max inaccuracy should not be greater than 1/100 inch (0.254mm).
That method doesn't give you lots of "off grid" error messages performing a design rule check and addionally the copper traces are straigth into the pad.
Having "constructed" a package with odd inch measurements, like 1/20 inch and you perform the DRC based on 1/10 inch grid you will also get "off grid errors". Use the smallest applicable grid size for the DRC. It helps very often to detect too small distances between a pad and a trace.