I'm not a fan of convection for cakes and brownies, in my experience it tends to dry them out or burn them, I would usually only use convection for brownies when I am concerned about consistent airflow. However, in your case when you want a nice and done outside with a gooey inside convection may work well, although you can use conventional mode with a slightly higher temperature to achieve the same thing.
Rather than looking at this from a time and oven temperature perspective I would suggest you go by internal temperature to see when it is done. I use a digital instant read thermometer to take temperature measurements, if it's below I keep it in a few minutes longer and check again. For gooey brownies I aim for a temperature of 85°C(185°F), give or take a couple of degrees C in the dead center of the brownie.
The benefit of this method is that you can use a range of temperatures and times but you'll always have the right gooeyness. You can then vary the oven temperature and mode to find what gives you the right outside result. Write down your temperatures and results and within a few batches you'll have it nailed.