Since I failed to complete this step (and have since come to the fail-point while trying to install Xorg with a Radeon HD 7950 (after many detailed/tedious customisations)), I was {almost} bereft at the prospect of having to "start-from-scratch" ;(
Finding that I had not installed 'sys-kernel/linux-firmware', I re-compiled the d-s-lts-4.9.168_p1 kernel "after-the-fact", did a 'ego boot update' and rebooted.
Apart from the fact that the 'ego boot update' re-set my Dell P2715Q to rendering tiny-text in the console, "startx" finally brought me to a successful TWM session ;)
Heartening to realise that I did not have to [entirely] Start-From-Scratch.
Regards, splifingate
