If you are looking to be in the sun at Miller Park, you will want to find yourself in the lower seating rows down the third base line. These seats will get the most sun of any in the ballpark when the roof is open, something which we experienced personally during a trip to the ballpark last summer (we ended up moving to the first base side since it got too hot for us).
For the best sun, look for the lower seating rows in Sections 124-129 as these are the best options for the most sun exposure during day games at Miller Park. Just be sure to bring your sun screen!

You can see the sun focused on seats down the third base line (taken from Section 404)