A rivet spacer was originally designed for evenly spacing rivets on aircraft, but it can be used for evenly spacing anything, from buttons on a sweater, to finish nails on those infernal Ikea drawers you are trying to put together nicely. It completely eliminates any need for calculations and weird fractions. You just stretch the fan to the end points with the number of holes you want. It is one of those tools that you never knew you missed until you use it once.
[The maximum span of the open spacer is 37 inches; when closed it collapses to 10 inches.]