Since your establishment is 40+ years old, had you complied with the provisions of the Payment of Bonus Act,1965 strictly, I think that this question would not have certainly arisen.
The minimum bonus and the maximum bonus contemplated respectively u/s 10 and 11 of the Act are strictly based on the allocable surplus in any accounting year only. But, in any already existing establishment which already crossed the infancy period mentioned in Sec.16, the calculation of allocable surplus for any accounting year should be made only with reference to the Set on and Set off of allocable surplus as provided for u/s 15 of the Act.
Therefore, in any establishment like yours, payment of minimum or maximum bonus for any accounting year has to be determined not only on the basis of the profit or loss in that year. For further clarification, I'd request you to go through sec.15 of the Act with a conjunctive reading of the illustrated calculation provided in Schedule IV of the Act.
19th April 2018 From India, Salem