Dimension with equations

I am trying to figure out an equation with mounting holes put in with the hole wizard. I want my front holes to be a half inch off the front edge of the mold except when the overall length of the mold is on a half inch increment. When the mold length is on a half inch increment then I want it to be one inch away from the front edge. An example is when the mold length is 20" the front holes will be a 1/2" off the front when the mold length is 20.5 the holes will be 1" off the front edge. Any Ideas?