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.