For most home cooks, using most home stoves, in typical quantities -- which includes YOU, yours, and yours -- time meat by the weight of the individual roast, not the total weight.
Thus, you're timing for 3.5#. However, make sure the meat is at room temp and your oven is THOROUGHLY preheated before putting the meat in.
Your old recipes called for approximately 35 to 45 min/lb. For a 3.5 lb roast:
- 35 min/lb is around 2 hours;
- 40 min/lb is around 2 hours and 20 minutes (h/t Mike); and
- 45 min/lb is around (wait for it) 2 hours and 40 minutes.
Use a thermometer. The meat's internal temp should be between 190 and 195F. If you don't have one, buy one. An inexpensive "instant read" is fine. If you want to keep an electronic probe in one of the roasts during the entire cook -- better still.
Even though your roasts will almost certainly cook at the 40 min/lb rate, use 3 hours (45 min/lb plus 20 minute rest) to figure serving time (and don't forget the time it takes to temp the roasts, preheat the oven; not to mention allowing time at the end to defat the juices for gravy, and slice and plate the meat). Brisket takes a long time to rest. When you remove the roasts from the oven, keep them tightly covered and you can rest for up to an hour without the meat getting too cold.
Do nott attempt to slice the brisket until it's well rested.
Brisket does not need to be "piping hot" at service.
You can use your knife to correct for minor imperfections in the cooking time. If the meat is a bit overcooked and wants to fall apart, slice thick. If the meat is a bit tough, slice very thin.
Always slice brisket across the grain.
Latke, not latka. Latka one of Andy Kaufman's characters on Taxi.
Otherwise, Ed was right.
A bi gezunt and shenah tovah.
Edited by boar_d_laze - 9/18/12 at 2:49pm