I am very new to this so I am not an expert and also haven't figured out timing for recipes. The one suggestion I do have, is to use a leave-in thermometer and go by the temperature instead of time. However, I read somewhere (don't remember where) then the temperatures for measuring doneness should be lower for grass fed beef than conventional. If anyone has a temperature listing please provide, because my preliminary search didn't get what I was looking for.
One tip about cooking for a temperature that is very important, is that internal temp continues to rise for a few minutes even after removing from the oven.