The exothermic reaction occurs when the two chemicals come into contact with each other. This can produce local boiling and this is where the chlorine can be produced.
You will have to judge the reaction for yourself. To get an idea pour the chemicals into a graduated cylinder and use your hand to determine the extent of the exothermic reaction. You can also check for any signs of boiling.
I don't end up with chlorine in the solutions that I make. I am using the chlorite - HCl method. As I mentioned earlier, I adjust the pH of the solution to near neutral.
If I needed 250 ml of 3000 PPM chlorine dioxide in water I would use the equivalent of 4 ml of Miracle-Mineral-Supplement in the process...
Keep in mind that it is difficult to measure chlorine in the presence of chlorine dioxide. It is much easier to remove the chlorine and check the differences in chlorine dioxide to see how much chlorine was present.