All defaults are invented: a module with Isc 10.4 A and Voc 45.0 V, a 20 A module maximum series fuse rating and a 30 A conductor limit. The factors 1.25 (backfeed), 1.56 (fuse minimum) and 1.2 (fuse voltage) come from the Wiles, Mersen and Eaton documents on this page. They are not the code text. Replace every value with your datasheets and your adopted code.
Design aid only. The fuse must also be listed for PV, rated for the DC voltage and accepted by your AHJ and the module maker. Mersen words the no-fuse exception with "or"; Wiles compares with both limits. This tool flags the string if backfeed passes either limit, and follows Wiles, who allows exceptions.