SPICE type CAD tools usually specify ideal transmission lines using the TD parameter. TD is the associated time delay of a signal transiting through a section of an ideal transmission line. However, sometimes it is easier to think in terms of physical length of the T-line. A calculator that does this is available from Signal Processing Group at the “calculators” menu item. Please visit that as well as a list of other calculators ( slanted towards RF/analog and microwave/mmWave design ) at the “calculators” menu item