As to each participant or as to all participants the stock purchase plan shall effectively limit the aggregate dollar amount or the aggregate number of shares of stock which may be allocated or which may be subject to incentive, qualified, restricted, or employee stock purchase plan stock options granted pursuant to the plan. The limitations may be established on an annual basis, or for the duration of the plan, whether or not the plan has a fixed termination date; and may be determined either by fixed or maximum dollar amounts or fixed or maximum numbers of shares or by formulas based upon earnings of the insurer, dividends paid, compensation received by participants, option prices, market value of shares, outstanding shares, or percentages of outstanding shares, or similar factors which will result in an effective and determinable limitation. Such limitations may be subject to any provisions for adjustment of the plan or of stock allocable or options outstanding thereunder to prevent dilution or enlargement of rights.