The value of desirability depends upon the data obtained form the experimental design. It is not mandatory but should be near to 0.9/1.0. If its not 0.9/1.0, then higher value of desirability should be considered depending upon the model and the optimization criteria .

yes i know but can you explain if i didnt get 0.9/1.0 then what is the criteria to get the highest, cat it be selected if the highest value is 0.5

If your desirability value is too low like 0.5, it means the desired response from your formulation is not achieved. You may try changing the independent variables or its values to get the desired response.