If memory serves the maximum length for one loop of 1/2" is 300 feet, (4) loops of 250 feet would be better. If you can layout your run lengths to be within 10% of each other you can eliminate the need for flow balancing. I took some extra time to layout my runs on CAD to optimize/equalize my loop lengths. I also layed it out so the hottest water ran through the perimeter tubing.
I'm not a radiant expert (although I did pretend to be one last year when I built my house) but I doubt if one crossover would hurt. It's not a "best practice".
