Copper is not opaque so the speed of light is copper is not relevent - you are reffering to the speed of electromagnetic wave propogation outside a copper wire.
No, I'm referring to the speed of electromagnetic radiation propagation (c, familiarly the 'speed of light') through copper. I.e. 'light' in this case meaning photons, for which copper is anything but opaque, given the appropriate frequency of photon. Likewise, 'copper' in this context could encompass UTP, STP or various waveguides with a diverse range of dialectrics. The formula is no less relevant and for the copper based transmission media 'about 75%' is a perfectly fine approximation.
Should have read copper is opaque. Copper obviously does not transmit light very well, it is mostly reflected or absorbed. I'm not saying the formula is not relevent, I'm saying that reffering to the speed of light in copper is pretty nonsensical when you are in fact meaning the electromagnetic wave propogation.