Broad-based weighted-average is the most founder-friendly variant of weighted-average anti-dilution, calculating its formula with a denominator that includes the broad fully-diluted cap table. That denominator (A) includes outstanding common, preferred on as-converted basis, outstanding options, warrants, and other convertibles, producing the smallest possible adjustment for any given down round and therefore the least dilution of common stock and founders. It is the modern venture default and the standard founders should insist on at every priced round.
The formula and the "broad-based" distinction: