If HP is the main reason for visiting Universal and you only have ONE day, visit HP first thing in the morning! Whenever I go, the parks tend to be open until 8 or 9, so the crowds start to die down around 6 or 7. Since the park is only open till 7, I have no idea if it will die down enough. It is much better to arrive at the time the park opens to enjoy HP for a bit before the crowds roll in than to wait and not get to see everything.
My plan of attack for you would be:
Arrive about 15-30 minutes before IOA opens. Do everything you want to do in HP and then explore the rest of the park. Hopefully you will be done around noon. Then, head over to the Studios and grab a quick lunch from a quick service place. Then, ride everything you want to experince in the Studios. Finally, around 5 I would head BACK to HP if you want to see it at night (or at least with the sun going down) and buy some items from the shops. This will give you about 5 hours in each park, which is not enough time to do/see everything. The T2 3D show alone takes about 30 minutes. Finally, after your day in the parks, go enjoy a nice sit down meal...just don't have a nice sit down meal while the parks are open!