Use the noun touchstone to describe a basis for comparison. For example, a filmmaker's touchstone might be her all-time favorite movie; she wants her movie to be that good or similar to it in some way.
Touchstone as it is defined today comes from an actual stone. In the late 15th century, gold and silver was rubbed, or touched against black quartz — the touchstone — to determine the purity of the metals. This was done by looking at the color of the streaks left on the stone. This is why a touchstone, in the figurative sense as it is used today, is a measuring tool.