As soon as practicable after the close of each fiscal year during its first 10 years of operation, each mutual bank shall credit to surplus at least 10 percent of its net earnings before interest for the preceding fiscal year. At the close of each fiscal year thereafter, each mutual bank shall credit to surplus a proportion of its net earnings for the preceding fiscal year, not exceeding 10 percent, as the department may prescribe by regulation. These credits to surplus shall be made only until surplus equals 12 percent of deposit liabilities. Thereafter and so long as surplus equals 12 percent of deposit liabilities the mutual bank may credit further amounts to surplus as it may determine.