If the box is unchecked (boolean value is FALSE), the dependent tool will not execute. You probably have to create another boolean parameter for calculate field. For the entire model to run, both are checked. When only calculate field should be run, Solve is unchecked and calculate remains checked. Probably not the most elegant solution but as far as I know that's the way model builder booleans work. They dont execute the dependent tool if false, period. Since your calculate is further down the chain, that one does not execute too.