Using Formula in the Predecessors column

I'd like to include a variable in my Predecessors column. So, for example, I might have a column "Contingency" and want the Predecessor formula to be "2SF-[contingency]" - where it takes the value from the contingency column (which may be 0 or may be several days).

Then, if someone updates the contingency column, it updates the plan.

Can this be done? Doesn't seem to work any way I try . . . .