How much white chocolate should I add to this mix?

I'm baking brownies for my best friend's birthday. She LOVES dark chocolate and white chocolate so I'm combining the two in brownies! I bought a pack of decadent fudge brown mix and a 12 oz package of white chocolate morsals. Now, I don't want to white chocolate to be too overpowering, but I definitely want it to be very noticable for her. What measurement of white chocolate chips should I add to the whole batch? I was thinking either 1/2 or 3/4 cup but I have NO idea! And do you think I should mix it in with the batter or pour it on top before I bake it? Or start baking the brownies for a bit and then take them out and pour melted white chocolate on top? Thanks!