It's recommended that half the height of the mold/tool is left between the mold/tool and the edge of the forming table. For example: If the mold/tool is 4 inches tall, a 2 inch gap between the mold/tool and the edge of the forming table is ideal. This recommendation is meant to maximize the distribution of plastic across the mold/tool. If wall thickness isn't a concern these boundaries can be pushed however, the object's walls will thin.