You can use the grid in the spline modeler with the "snap-to" enabled for precision. As for the boolean operations -- they're a mess in most programs. (Bryce being the one exception, because its booleans don't really change the model's geometry, but rather just changes how the model renders -- kind of a clever approach, but useless in a geometry-based renderer like Carrara.) The only suggestion I can make is to be sure your surface fidelity is as high as you can crank it for both objects you're performing the boolean on. I've managed to sneak out some decent booleans in the past (mostly in RayDream, but a couple in Carrara, too), but I don't count on it being possible. Hope this is helpful.