I suspect my question really is, "can I calculate the CR rating for a player character generated via the PHB rules for the purposes of determining combat/encounter difficulty for a party?" A follow up question would be "How can I scale an NPC or monster up or down in difficulty".
I have not found any direct rules to convert a player character into an NPC and calculate the CR for the purpose of combat. In the DMG, there are rules for creating NPC monsters (p.273) and rules for calculating encounter difficulty (p.82). However, if I have a level 5 wizard player character that was generated by following character creation in the PHB, that becomes an NPC that the players decide to attack or is wanted to be used as en encounter, is there a way to calculate the CR of this NPC and apply the Encounter calculation tables to determine difficulty vs. the party strength?
Commenters may (rightly so) see my question as a duplicate, as there are similar questions:
- Why shouldn't I use fully statted PC characters as NPC antagonists in D&D 4E?
- Determining "level" of an NPC ally for purpose of budgeting encounter XP
- If I design a PC for use as an NPC, how much XP should he give when killed?
However, the answer seems to be that there is no direct way to calculate the CR from a player character generated by the PHB rules. The most accurate way to generate and size a foe seems to be to go through the process of generating from scratch using the Creating a Monster guidelines and sizing accordingly.
I also used the Monster Manual's NPC section on Mages (MM p.347), Priests (p.348) and Veterans (p.350) as a reference to design. I could not find rules on how to scale these up or down in hit dice and AC to tailor them for a party of a certain level. (How to scale down Princes of the Apocalypse for a 3-adventurer party?).
Maybe the real answer is always to generate an NPC with the traits of what a Nth level PHB character would have, and size according to DMG p.273?