A meat thermometer can help with more precise cooking times. Don’t let pride stand in the way of getting one. To use one properly ensure that it slides into the thickest part of the cut. Also, bear in mind that the temperature will rise once the meat has rested.
A heat guideline for your thermometer:
These indicate the meat’s temperature as it comes out the oven; the readings will increase by about 7°C, once the meat has rested. Also bear in mind that temperatures vary for different types of meat.