Custom column based on slicer values

The selections in the slicer doesn't affect a calculated column. As with calculated table, it gets updated only when:

• The related tables are refreshed
• Upon creation
• When the formula is updated.

Instead of calculated column use a measure.

PC Count = 
CALCULATE (
    COUNTROWS ( Computers ),
    FILTER ( Computers, Computers[PC_Purpose] IN VALUES ( PCPurpose[PC Purpose] ) )
)

The measure above will return the same result whether there is a relationship or not between PCPurpose and Computers tables. If there is an existing relationship, you cause simply COUNTROWS the Computers table.