Dundee,
I've done quite a few legs. On a Cookshack Smokette or 50 Series, if you do low and slow, I'd set the thermostat at 180 and take it to an internal of 150 - 152 (my wife doesn't like it rare). I usually buy boneless legs from Sams Club, which are 1/2 lb to 1 lb larger than the size you listed. My Model 55 seems to take about 7 hrs to get to the above temps on 2nd rack from the top. With your 3 1/2 lb'er, if you use a remote thermometer and keep your door closed, you should probably be done in 6 hrs. If you're a few degrees short of your target temp and everyone is stomping their feet to eat, it isn't against the law to give it a crank up to 250 to bump it up the last few degrees. Let the lamb rest 15 before cutting, or your going to have a huge pool of juice.
I hope that helps.
A simple rub of Tones Rosemary & Garlic, a little garlic/rosemary/red wine injection (optional), and a hollandaise sauce over the sliced lamb makes one fine entree. Try it sometime.
Paul