"Earned premium" means the total gross premiums which become due the insurance company, without reduction of any kind, except for premiums refunded or adjusted on account of termination of coverage, appropriately adjusted for changes in unearned premium reserve calculated as follows:(1) for single premium term life coverage, by the single premium for the remaining term and benefits (rule of anticipation);(2) for single premium uniformly decreasing disability insurance, by the mean of the "sum of the digits" (rule of 78) and the pro-rata methods, or at the option of the insurer, by the "rule of anticipation";(3) for other coverages, the "rule of anticipation"; and(4) by another method which produces substantially equal unearned premium reserves and is approved by the commissioner of insurance.