I have done this for some classes and levels; here are my results, and an example of my process.
[sblock="Wizards"]Wiz 1 - CR 1/2: DCR 1/4, OCR 1
Wiz 2 - CR 1: DCR 1/4, OCR 2
Wiz 3 - CR 1: DCR 1/4, OCR 3
Wiz 4 - CR 2: DCR 1/4, OCR 4
Wiz 5 - CR 5: DCR 1/4, OCR 11
Note that the character is still Defensive CR 1/4 at 5th level. I was using the spell list from the MM "mage," which includes mage armor, but not shield, and no relevant 2nd-level spells. Fireball is fireball, but this dude won't last one round in a straight fight. The Eternal Flame Priest from Princes of the Apocalypse is a good firebally guy (though it has the same AC and twice as many HP as my wizard stat block, and is listed as CR 3. Hmm....)[/sblock]
[sblock="Fighters"]Ftr 1 - CR 1/2: DCR 1, OCR 1/2
Ftr 2 - CR 1: DCR 1, OCR 1/2
Ftr 3 - CR 1: DCR 1; OCR 1
Ftr 4 - CR 2; DCR 2, OCR 2
Ftr 5 - CR 3; DCR 4, OCR 3
Ftr 6 - CR 4: DCR 4, OCR 4
Ftr 7 - CR 4: DCR 4, OCR 4
Ftr 8 - CR 5: DCR 5, OCR 5
Ftr 9 - CR 6: DCR 6, OCR 5
This is for a sword-and-shield Fighter with all the simplest options. Stats are the standard array with no racial modification. Armor is upgraded at higher levels, but no magical items. Any of these factors can potentially change the CR.[/sblock]
[sblock="Two Examples"]
Note: These examples use a homebrewed race and futuristic weapons, so these results are not correct for PHB characters. Maybe I'll post the stat blocks in the homebrew forum, but this is just for an example of the process of applying the monster math to characters.
The Maverick is an 11th-level Fighter with the following options:
Stats before racial adjustment: Str 15, Dex 15, Con 15, Int 8, Wis 8, Cha 8
Level 1: Great weapon fighting
Level 3: Champion
Level 4: Great weapon mastery
Level 6: +1 Str, +1 Con
Level 8: +2 Str
Level 10: Defense
Before calculating CR, I have to do some math regarding the Great Weapon Master feat.
First of all, should the Maverick use regular attacks or power attacks? Assuming 3 attacks, regular attacks give you 90 DPR at +9. That's OCR 14. Power attacks give you 120 DPR at +4. That's CR 19 damage, modified down for the low attack bonus, yielding OCR 16. Is a power attack really 2 levels better than a regular attack? I'm not sure, but the versatility seems worth it. I'll calculate as if the Maverick is always using power attacks.
Second, how often will this fighter get to cleave? Assuming one use of Action Surge, the Maverick will attack 12 times. Based on a formula that I found on some forum [(18/20)^12], the Maverick has about 71.6% to get a critical hit. Add the possibility of knocking down one of the PCs or their allies, and it seems safe to assume one cleave attack during the first 3 rounds.
So, 13 attacks, for an average of 173.3 DPR at +4. That translates to OCR 19. Damn.
Of course, that's assuming the Maverick is attacking with its sword every turn. If we restrict it to the antimatter rifle, it's 12 attacks, for an average of 64 DPR at +7. That's OCR 10.
As for defenses: AC 19, 155 hp. DCR 8.
Total CR 13 in melee, 9 at range.
***
The Hunter is an 11th-level Fighter with the following options:
Stats before racial adjustment: Str 15, Dex 15, Con 15, Int 8, Wis 8, Cha 8
Level 1: Archery
Level 3: Champion
Level 4: Sharpshooter
Level 6: +1 Dex, +1 Con
Level 8: +2 Dex
Level 10: Defense
Assuming one use of Action Surge, that's 12 attacks in the first three rounds. Power attacks are better than regular attacks, so I use those; 112 average DPR at +6 gives me OCR 16.
Of course, that's assuming it's using the laser rifle. If we're talking melee with beam shortswords, we're talking 14 attacks, for an average of 66.6 DPR at +9. The DMG gives me OCR 11.
Assuming Second Wind and full regeneration, the Hunter has 155 hp and AC 19. DCR 8.
So, all together, that's CR 12 at range, and 9.5 in melee.[/sblock]