Can they just set toe less negative to compensate? What car is it?
They can compensate if they know how much to compensate for. But usually they just go by what the machine tells them to do, which would be applying the BMW's weighted car specification to an unweighted car.
Mine is an E46 coupe. Anyway, I did go to RSK. It was cheap and cheerful. They didn't bother checking tire pressure etc. The guy said it would take 45 minutes but it took 10 minutes to adjust my front toe. He wanted to change my rear as well, but I didn't allow him. I had been happy with my rear tire wear for the past 5 years and saw no reason to make a change.
It was easy and quick money for them, and I found the result acceptable. Since I usually drive alone anyway, aligning the the car for an unweighted set up isn't a problem. If I want to, I can increase toe-in a bit whenever. After all, I have been attempting a DIY front alignment for the past 12 months and so am quite experienced.
As I didn't have a reference point for what the roughly correct alignment measurement using the string method should be, my DIY attempt failed and worn out an old set of front tires that had a nail in them. Having to pass MOT meant I had to get a new set sharpish, and the alignment was to avoid an immediate wear on the new tires.
Now that I know what the measurement is for the roughly correct alignment, my future DIY attempt will no doubt succeed. It will be whatever I have now plus a tiny bit more toe-in - 1/16th of a turn on the tie-rod to shorten it per side perhaps.
As for RSK, they do have a hunter machine, and I believe their guy will accommodate special requests if you are specific, such as: give me a little more toe-in to the max of the machine indicated range will ya.