Should we create a new constant or modify an existing one
It depends on the meaning of this constant. If the value ROW_LOC - 1 has logical relation to ROW_LOC (e.g. it means that it should go right before ROW_LOC in some list) then just use ROW_LOC - 1 there, but if it's just 0 - then either define a new constant or just use 0 as value, depending on situation.

What I would suggest is to rename ROW_LOC to more meaningful (descriptive) name unless it's a well-known acronym in your company.