Adler reports an identical entry in the unedited Lexicon codicis Barocciani 50.
A TLG search shows no other attestations of the word, which is presumably a transliteration from legal Latin. DuCange s.v. says it comes from Latin formula, and cites Ammianus Marcellinus 14.1.3: oblato pretioso reginae monili id adsecuta est, ut ad Honoratum tum comitem orientis formula missa letali, homo scelere nullo contactus idem Clematius nec hiscere nec loqui permissus occideretur ("she presented the queen with a valuable necklace, and thus secured the dispatch of his death-warrant to Honoratus, at that time Count of the East; and so Clematius, a man contaminated by no guilt, was put to death without being allowed to protest or even to open his lips.")