How to estimate microwave lamda on Si-SiO2 Substrate

Many literatures offer a crude rule to decide whether a microstrip can be treated as lumped or distributed: length < 1/10 lamda of wave on Si-SiO2

there exist several modes when wave propagate on Si-SiO2 substrate. some papers give detials analysis. This substrate is different from tranditional microstrip on dielectric material in common text books.

But do people use some easy rules for hand calculation to give a hand caculation of lamda, for example, 10Ohm*cm Silicon substrate with 2um thick SiO2?

Thank you!