[en] As soon as two SLC beams make it to the intersection region, both transverse offsets, spot sizes and shapes can be extracted from the pattern of angular deflections produced by the electromagnetic interaction of the two beams, as one is scanned across the other. These deflections, measured in two high resolution Beam Position Monitors (BPM) mounted symmetrically on both sides of the intersection point, will produce detectable signals allowing spot sizes to be tuned, even with the very low luminosities expected at turn on. They will also furnish a good signal to monitor beam centering and will therefore become an important part of the FFS feedback system. This note summarizes the formulae which will allow us to correlate BPM offset readings with the properties of the two beams, and describes the range and limitations of the technique in the case of SLC