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Calculating in what way? I'll write some general rules/guidelines:
Monsters have their CR-value listed in their statblock.
For multiple monsters of the same CR, an easy way to determine encounter CR is to add +2 to the CR for each doubling of the number of monsters. So, two CR 5 monsters make up a CR 7 encounter (5+2), four CR 5 monsters make up a CR 9 encounter (5+2+2), eight CR 5 monsters make up a CR 11 encounter (5+2+2+2), and so on.
For multiple monsters of varying CRs, the easiest way to determine encounter CR is to add up the XP values (not the CR values), and comparing the resulting total XP to the XP chart to find the equivalent CR.
NPCs with PC-class levels (regular classes) have a CR equal to level-1.
NPCs with NPC-class levels (warrior, expert, adept, etc) have a CR equal to level-2.
NPCs with PC-class levels, PC-wealth, and PC-stats (which would also apply to PCs) have a CR equal to level.
A party built with 25 point buy might be expected to handle CRs increased by 1, compared to a party built with 15 point buy.
That character presumably doesn't have PC-wealth, so it falls under the line I wrote slightly above the one you quoted:
"NPCs with PC-class levels (regular classes) have a CR equal to level-1".
When CRs drop below 1, then each CR-reduction will reduce the CR by fractions rather than dropping to 0, so "level 1 minus 1" becomes CR 1/2 rather than CR 0. If reduced by another 1 (as it would if the NPC had an NPC-class level), the CR would become 1/3.
An NPC with class levels is considered a Challenge Rating equal to his class levels -1. So your 1st level wizard is a CR 1/2, a 2nd level human wizard would be a CR 1, a 3rd level would be a CR 2, etc.
I suggest reading the NPC chapter in the core rulebook, and reading up on NPCs in the Gamemastery Guide to get a real feel for all of this.