I've found there is an inverse relationship between doneness and heat of the pan. Remember that you are searing the steak, the faster it sears the faster you can flip it over. So the rarer you want the steak the hotter the pan should be.
Some people prefer to use separate cast iron pans for the very hot tasks as it can destroy the seasoning, not the pan, just the seasoning.
If you don't have a good exhaust system then open the windows before you begin cooking.
Season steaks with salt, garlic salt, and a great deal of freshly ground pepper. Preheat oven to 350. Heat cast iron skillet very hot, then place steak in skillet. Cook on very high heat for 3-4 minutes, depending on thickness of steak. Turn over -- a very nice crust should have formed. Heat on second side for 1 minute. Then take pan and place on lower rack of oven and turn heat up to 450. Wait 5-8 minutes, depending on thickness.. Steak should now be a good medium-rare.