I honestly think starting with an unoptimized poorly played Rogue is a good first step.

Short sword, melee attack, no advantage, but does have a (otherwise not helping) ally enabling sneak attack.

Then add in minor optimization. Dual wielding rapiers, uses uncanny dodge. See how many baseline rogues it can kill in a row.

Then add in heavier optimization. Optimal subclass, SCAG cantrips, uncommon gear in T2, rare in T3, very rare in T4.

Run each against the previous versions in "1 at a time, but with the assumption aoe damage isn't wasted and you get an ally to enable sneak attack" fights.

You can then factor in short-rest resources (in rogues case, usually just HD) and long-rest resources (usually HP and HD).

I'd peg a "L X monster" as a part way between an unoptimized and minorly optimized power level. Design wise, I'd have "role" based power boost that makes up ~25% of the power budget.

Note that the general power level of a creature is (offence) times (defence) in a typical fight. This is quadratic, so it might be a good idea to take the square root to make it linear. (If you make one creature with the O+D of two creatures, you get a creature with 4x the power if you just multiply them; hence, quadratic. The square root is a first approximation that keeps it linear; this isn't quite perfect, as one creature with 2O and 2D is different than two 1O 1D creatures in a few ways; it has CC problems, it is stronger against AOEs, and against focus fire it deals 33% more damage in a fight; arguably 2x 1O1D is power 3, while 2O2D is power 4.)