I mean, the reference images are still okay for getting "flat"ish parts of the face somewhat close to correct like eyes and lips from the front and the ears from the side, but parts like the nose, cheeks, neck or the general shape of the head is somewhat tough to nail down as they are the biggest victims of perspective distortion.

Especially the jaw area seems to look quite warped and inaccurate when you switch to perspective after aligning it in the orth front and side view.

So, so far I've also gone with "guess work" / eyeballing it using other reference images that i have open on another monitor. But if you are trying to model a likeness at least somewhat accurately, that seems to introduce a lot of potential errors as even the faintest inaccuracies can make it look somewhat "off".

Thought there might be some techniques out there that make it a bit easier to work with non-ortho reference images without resorting to just 3d scanning or taking lots of references with a strong tele lens.