Yep, go by the internal temp. of the meat. However - don't get it too hot or it will over cook and dry out. I go for 160. The
USDA says 145 is safe.
In a dutch oven though, it doesn't seem like a linear relationship between time and temp. The temp will start rising, then seems to hesitate around 130 to 150. Be patient and keep the coals coming. Once it gets over this "hump" you're good to go!
Don't be afraid to take the top off. The dutch oven retains heat pretty well, just don't over do it. Do your fist check about three hours in. And don't dump your coals on the meat!
BTW - there is an easy to remember formula for knowing how many coals to put on your DO. Take the diameter (in inches), subtract 4 for the bottom, add 4 for the top. So if you have a 12" DO, that would be 8 on the bottom, 12 on top. Good starting place.