In your Designer Help database, go to the Index, down to Column, choose the option "programmatic name for", then choose "Advanced options for columns".
In that article, it is said that "If you first define a column using a simple function, then you create another column that depends on the value of the first, you must edit the programmatic name of the first column to something other than a dollar symbol ($) and a number".
In other words, to do column formulas that depend on the result of other columns, you must assign a name to the column that is the source for the other column.
Then you can do your calculation.
Pascal.