How do I figure out the challenge rating for a single pc? (D&D 3.5)

Do you mean the threat of a single PC, or do you mean creating a challenge for a single PC?

Both.

I want to know what the threat of a single pc is,

and I want to make sure the CR calculator I used is correct...

The CR level of a single creature is actually in the monster manual. And it speaks about the CR of a character that has either npc levels, or character levels. I believe the general rule is CR= Level -1... You'll just need to look it up in the back of the monster manual... I do believe. It's been awhile.

Technically it should answer both questions.

I already looked there and didn't find the answer.

It just said that parties with more people can take higher CRs, and parties with less take a lower CR.

And that when monsters take pc classes the CR depends on how the class effects it's existing abilities.

This is where you need to look.
It explains it better than I can.

To save you looking, if the creature is plain jane, with no ECL (Effective Character Level), then his CR will be Level -2(for NPC levels), Level -1(for PC levels).

When a creature has HD, or ECL attached to it, and you are getting into the higher levels, you need to adhere to the concept of Associated class levels, and nonassociated class levels.

It's all there. It's chapter 3, under Advance Monster Challenge Rating. Hope this helps.

I'll read through it again.

Thank you for your help.