Can I and how do I calculate time
Are you just slow roasting until they're tender? If they're chuck roasts, I generally roast at 300 - 325F, and I start checking at 1.5 hours (it can very often take up to 3). Don't check for temperature, but make sure a knife can easily separate the meat. Just allow for time and make sure you have 3 - 4 hours to cook. Its easy to keep a roast warm, but its difficult to rush the process.