Baking – How to estimate the time required to bring a piece of meat to a certain temperature

The first question I would ask yourself is, why do you want your steak to be 170 degrees? The temperature and "well done" is just an arbitrary measurement and you really should look deeper to get a better answer. I see a few potential answers to that question.

1 - You are worried about getting sick from under cooked food

If this is the case, you really don't need to be as worried about steak. But, if you are still worried, cooking longer never hurts to kill more stuff. The problem is you will find your steaks to generally by dry and not very flavorful (or maybe too flavorful if you get burn it). Cooking the steak slowly (at a lower temperature) will help to keep the steak a little juice while getting it to be well done. You might also want to get over being scared of getting sick and move on to option 2.

2 - You don't like steaks that still show some pink

If this is the case, then maybe some experimentation is needed. Cut your steak once it hit 170 degrees. If you don't like the look, leave it on a bit and know that in the future you need to let the steak sit for a bit longer. If you like the look, then take it off. After a few times of repeating this process, you will learn what you like. Remember the end goal is to get a steak cooked how you want, not reaching a specific temperature.

You will get to the point that you no longer need to cut your steaks to check them. You should also keep in mind that steaks will cook for a bit longer after removing them from heat. You may end up with a slightly pink steak during this trial and error process, just use it as a learning experience.

3 - You pride yourself in being able to grill, but you received a request from a friend for a well done steak

In this case, you don't have the ability to perform experimentation over time and may very well be scared to cut a steak before serving it to some one. Get over yourself and be willing to tell your friend that you are inexperienced with well done steaks and ask for some help telling when it is done. They may not be experienced themselves and will end up just cutting the steak to see where it is at anyways.

There's a few things that could be going on here:

• cut: not all steak cooks at the same rate. Really tender cuts cook faster than some of the tougher, more flavorful cuts, increasing cooking time by up to 50%. Cuts like tenderloin, filet, and loin (US) - Sirloin (UK) are more tender and cook faster. Denser cuts like sirloin, top sirloin, and bottom sirloin (US) and rump (UK) cook slower. If you are cooking a denser cut then you simply need to cook it longer.
• Pan heat: Most chefs have really good stoves that produce load of heat, most mere mortals have average stoves which aren't as powerful. Medium heat on a professional, or very high quality stove is hotter than the medium heat on an average stove, so try cranking it up to full blast. Also, make sure your pan is fully up to temp. I cook my steaks on a cast iron skillet, and I let it heat up for 10 minutes before I start frying steak
• Heat contact: even though steak is full of fat it takes some time for this to start working, so coating your steak with a bit of vegetable oil (not olive oil, it burns at high temperatures) will make sure it gets good heat contact

So my advice would be to get the pan hotter and cook it longer. Try adding one more minute per side.

EDIT: @kenny says that he is cooking loin, so assuming it's 3/4", or 2cm thick and looking for medium done-ness I would cook the first side for 5 minutes and then the second side 3 minutes. The uneven times are to make sure it cooks evenly. So you need a total of 8 minutes cooking time. I don't do the flip every minute method because the uneven times work for me, and it lets me do other prep.