Flat iron steak is very consistent in size so once you cook it you’ll pretty much know how long to get it where you want it on the grill. Depending on how it’s butchered, sometimes there will be a length of connective tissue that will look like a white line running through it. Look for one without it, or trim it out. This marinade has no acid in it so you can let it marinate overnight if you’re a planner.