Maybe this is obv but I just realized that if you want exactly a one-in-a-hundred chance, it's 19 or 20 on an unmodified d20 with disad. If you want exactly one-in-a-million chance, it's doing that three times in a row. If you want a (Edit:
this link is very spoilerific) Pratchett-style one-in-a-million chance
it's a 3+ on a single d20.