Column Calculated using Slider Input

Calculated columns are calculated at load and not affected by slicers and report interactions. You need a measure.

Y = 
VAR CurrentXValue = MAX('Table'[X])
VAR Result = CurrentXValue - [Selected Value]
RETURN
    Result

You can see my PBIX file here. 

Calculated columns appear to be very Excel-like, but they are anything but, and should generally be avoided. There are times to use them, but it is rare. Getting data out of the source system, creating columns in Power Query, or DAX Measures are usually preferred to calculated columns. See these references:
Calculated Columns vs Measures in DAX
Calculated Columns and Measures in DAX
Storage differences between calculated columns and calculated tables
Creating a Dynamic Date Table in Power Query