A little about time selection…
‘Doneness’ is a function of temperature. So it is a matter of selecting a temperature that will produce the finish that you like. For your medium rare 129F is a good starting point.
Time is more flexible.
Firstly, you need to cook your steak long enough for the centre to reach the desired temperature (which is where the thickness comes into the calculation). The warmth moves through the meat at approximately 1/2 inch per half hour. So the minimum suggested time for a steak would be an hour for one an inch thick.
Secondly, you’re also looking to give some time for the temperature to have an impact on the tenderness of the meat, allowing some time for enzymatic reactions and for collagens to break down and convert into gelatin. This is less important with a naturally tender cut like those traditionally used for steak, but most meats can still benefit from a bit of time. Collagens will start to beakdown noticeably from about 6 hours onward.
The time selection will be based upon the desired finished texture. A steak is usually cooked to a ‘chewy’ kind of texture with only minimal breakdown of the fibres, so you want to be under the 6 hour mark.
The temperature selected for steak is often under the temperatures required for pasteurisation, particularly for a finish on the rarer side of the scale. For this reason, it is best to keep your steak cooking times below the 4 hour mark.
As ACS points out there are some proteins which behave a little differently to these rules. Fish and eggs are much more impacted by time at temperature than most other proteins.