Cecilia,
The sewing machine belt should be just tight enough that it doesn't slip and no tighter. Generally when they are at that point, they look and feel like they are too loose.
You said rubber belt, if you are referring to the round rubber band type belt, they are junk and prone to slipping no matter how tight they seem to be.
I have also found that the amber cogged belts don't grip like the black V-belts do.
As for the motor position, as long as it does not interfere with anything I simply set them where the belt tension is right.
Joe