As mentioned in the accepted answer, the DMG offers a table with appropriate AC corresponding to the challenge rating (CR) of a creature. However, the question mentioned that this AC should be used as a reference for the sake of comparing character builds. Therefore, an average/reference AC for a given level of a PC could be useful (even though the question specifically asked for CR). Now the correspondence between the level of a PC and the CR of a typical enemy at this level is unclear (see discussion here), the DMG just mentions that
especially at lower levels, [the DM should] exercise caution when using monsters whose challenge rating is higher than the party’s average level.
When it comes to comparing different builds mechanically, I would suggest to use the following formula to determine a reference enemy AC for a given PC level:
AC = 8 + PB + AM
where PB denotes the proficiency bonus of the PC and AM is the ability modifier that any PC focusing on increasing a certain ability score usually has at that level. That is, AM=3 at levels 1-3, AM=4 at levels 4-7 and AM=5 at levels 8-20 (starting with a score of 16 and taking ASIs at levels 4 and 8).
Advantages of this way to determine the AC are that
- for calculations, the proficiency bonus cancels out (as in the formula occuring in the question),
- this formula also determins the spell save DC of a spellcaster, so there is a nice correspondence,
- this gives the same ACs as the ones given in the accepted answer (assuming CR=PC level and except for level 9) and
- any PC that increases its ability score used for attack rolls as soon as possible will have a 65% chance to hit against the reference AC at every level; this might simplify calculations.
Note that there is also the approach to fix the "baseline-chance to hit" (which implicitly determins the reference AC) and (as just mentioned) the formula above corresponds to fixing this baseline to 65%. Another prominent choice is fixing it to 60%, corresponding to the formula AC = 9 + PB + AM that would yield an AC of an enemy with CR slightly above the PC's level.