Disclosed is a method of determining a characteristic of interest, in particular focus, relating to a structure on a substrate formed by a lithographic process, and an associated patterning device and lithographic system. The method comprises forming a modified substrate feature on the substrate using a corresponding modified reticle feature on a patterning device, the modified substrate feature being formed for a primary function other than metrology, more specifically for providing a support for a vertically integrated structure. The modified reticle feature is such that said modified substrate feature is formed with a geometry dependent on the characteristic of interest during formation. The modified substrate feature can be measured to determine said characteristic of interest.