On a road double crank, the bottom bracket spindle aims to put the chain directly between the two rings. In doing so, the chainline is off by 1.5mm on either ring. Typically, you can use the same bottom bracket spindle intended for a double setup, or you can subtract 1-2mm and put the chainring on the outside of the crank spider.

For example, my old Shimano 105 road double crankset requires a 107mm BB spindle. A 103mm spindle would bring both crank arms 2mm inboard, which would likely give me a near perfect chainline if the chainring is on the outside.