My recommendation is to use tiny sections, like really really skinny sections. Another tip is to keep your hair "organized" when your flat ironing. What I mean is to keep the hair from the other sections that you're not working on out of your way.
I comb my hair and decide where my part will be, then I part my hair straight down from the end of that part to behind my ear. Then I comb all the other hair away and put it in a bun in a scrunchie on the other side of my head. I make a tiny horizontal part above my ear, put the rest of the hair in a clip and then flat iron. Be sure to get the flat iron as close to the scalp as possible.
Keep going until you reach the top of that section. Take down the rest of your hair, give it a quick comb, and make a new vertical part that ends behind your other ear. Put that hair in a bun, and flat iron the back starting by your neck.
I'm not criticizing you, but 420 sounds so hot to me. I set my Sedu to 340, I used to use it at 370 before I joined LHCF, and turned it down because of all the comments here. It comes out just as straight and just a teeny bit less shiny.
One last tip, I prefer ionic tools because they make my hair shinier, so I use an ionic blow dryer and flat iron. Also my blow dryer is 2000 watts, it gets my hair dry really quickly and also straighter so that when I flat iron my hair comes out straighter. I went to visit my sister once and forgot to bring my blow dryer so I had to use hers. She has a 1875 watt conair not ionic. It took forever to dry my hair, it wasn't as staight when I finished and my flat iron results didn't turn out as nice as they usually would.
HTH