You'll get the most accurate results by using a (affiliate link) Meat Thermometer. Cook to internal 125 degrees F for rare, 135 degrees for medium, 145 for medium well and 150 degrees F for well done. If you let the steak rest in cast iron skillet, the internal temperature will continue rising so you might want to remove the skillet from heat about 5 degrees before it reaches the desired temperature. My rule of thumb for when I don't have the thermometer available is using the way the steak looks plus I time how long it's been cooking. Again, this is an estimate and it's only a guide. It needs the cast iron skillet to be heated well before you add the steak in and the steak is about 1-1.5 inches thick, remember to flip every minute and baste with butter in the end. Usually this guideline works for me and I adjust as needed: - 5 minutes gets the steak to medium rare - 6 minutes to medium - 7 minutes to medium well - 8 minutes to well done