Clue: … 80% of that is due to 19 13 or 15 (4)
Answer: IDES
Explanation: Definition: 19 13 or 15
i.e. *id es[t] *('that is') using the first four of the five letters ('80%'). In the definition 19 refers to the clue 19A DAYS, but the 13 and 15 do not: in the Roman calendar the ides (strictly a plural, agreeing with 19A) were either the 13th or 15th day of the month. The Ides of March (or May, July, October) would be the 15th.