Is there a scale for measuring level of roast?

There are two factors here: surface treatment and doneness. Doneness (as mentioned above) is a factor of temperature and time. Surface treatment, browning, and crust, may be more what you're asking about.

There are simple ways to measure surface browning:

• colour (pink, grey, brown, freckled brown, and black)
• dryness (wet -> dry)
• thickness (thin -> thick)

So a good BBQ chunk of meat may have a dark colour (lots of browning, some burning) and a fairly wet and thick crust. It's wet, in that it's not crispy or crackled, and it's thick as in it's described as "bark".

A blackened, pan-fried steak will have a thin(ish) dry black crust, but show very little grey, assuming something like rare or medium rare.

A BBQed (not grilled) steak will have a slightly thicker, wetter crust.

A good smash burger will have a thin, fairly dry, and brown crust (possibly speckled).

A round/inside roast will be in between somewhere (probably), and can side anywhere from brown to dark, but usually fairly rare inside (up to medium).

A pot roast may be dark, but will almost always be wet and basically be all "crust" (or all one colour). Sometimes you brown pot roast ahead of time, in which case the surface may have some colour too.

A prime rib will have a wetish and darkish crust, and be very rare inside (ideally).

You may also consider the seasoning (and depth of seasoning) of a roasted food. BBQ often has a thick bark that is also seasoned most of the way through. Something like corned beef is cured, so has seasoning throughout (and on the surface).