Calibrated stones are cut to exacting specifications and snapsettings are made for calibrated stones. For example, this means that any faceted calibrated 6mm CZ or comparable stone will fit into a 6mm snapset. If for some reason a prong is out of alignment, which sometimes happens, it can be bent back into place before setting the stone. If a stone is only a smidge (technical term-lol) larger or smaller, the prongs can be bent in or out accordingly. Be careful. Stones can fall out of setting that does not hold the them tightly enough.
Adding snapsets to designs is a great way to showcase birthstones. To avoid having to remove stones to place them elsewhere, audition the stones before setting. Stones can be set on top of the settings, but putting them beside the settings or by the finished piece will work just as well to see if the combination needs adjustment.