A few years ago ISNA and FCNA came out with a strange position on how to set the Islamic calendar. Traditionally, the only two opinions have involved actual sighting of the moon to officially declare the start of the new month. From what I understand, the FCNA opinion substitutes the sighting requirement with calculations (as opposed to simply supporting sighting with calculations, which is totally fine).
Is my understanding correct, and if so, what is the evidence they present to support this opinion?