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value of guaranteeing a business loan
You are confining the way you and the other co-founders are paid for guaranteeing the loan to capital shares. Trying to determine payments by equity distribution is hard. It is a practice that many small companies particularly the ones in their initial stage fall into. I always advise against trying to make payments with equity, weather it is for unpaid salary or for guaranteeing a loan such as your case. Instead of thinking about a super sophisticated algorithm to distribute the new shares between the cofounders and the new investors, given a set of constraints, which will most probably fail to make the satisfactory split, you should simply view the co-founders as debt lenders for the company and the shareholders as a capital contributor.  If the co-founders are treated as debt lenders, it will be much easier to determine the risk compensation for guaranteeing the loan because it is now assessed in monetary units and this compensation is equal to the risk premium you see fit "taking into consideration the probability of default ". On the other hand, capital contributors will gain capital shares as a percentage of the total value of the company after adding SBA loan.