Figures are per worker; the number of workers only scales the payroll and the premium. The rate comes from the particular conditions of your policy, not from the law. Leave the declared pay equal to the real pay if there is no under-declaration.

With the declared pay equal to the real pay, the insurer answers for the compensation in full.

The rate is not in the law: Article 81(2) requires premiums to be graded according to the degree of risk of the activity and the prevention conditions, so the insurer sets it. Stamp duty and parafiscal charges are not included.