Delay of a trace from its effective dielectric constant and length, the maximum length mismatch allowed by a skew budget (for DDR, parallel buses or differential pairs), and the wavelength on the board at a given frequency.
Delay. A signal travels at v = c/√εeff, so a trace of length L has a delay of L·√εeff/c. In FR-4 stripline this is about 6.9 ps/mm (175 ps/inch).
Length matching. The maximum length mismatch for a skew budget tskew is ΔL = tskew·v. Use the effective dielectric constant of the layer where the traces run: microstrip and stripline propagate at different speeds, so a match on one layer does not hold if one trace changes layer.
Wavelength. λ = v/f. Quarter-wave lengths matter for stubs and resonances, and one tenth of a wavelength is a common limit for treating a structure as electrically small.
Part of the microstrip field is in the air above the board, which lowers the effective dielectric constant and makes the signal faster than in a stripline.
Take the timing margin from the interface specification. For example, a skew budget of 5 ps in FR-4 stripline allows about 0.7 mm of mismatch.
Bandwidth of a digital edge and the trace length above which it needs transmission line design.
Near-end and far-end crosstalk between two parallel traces, from the coupled-line impedances.
Width a trace needs for a current, or the current a trace can carry, with IPC-2152 and IPC-2221.
Impedance of an outer-layer trace over a plane, with solder mask or embedded, and width for a target impedance.
Impedance of an inner-layer trace between two planes, symmetric or offset, and width for a target.
Differential, odd, even and common-mode impedance of edge-coupled microstrip and stripline pairs.