The standard goal of valuing anything is to seek the fair price for that thing in the open market. Depending on what is being valued, that may or may not be an easy task. eg: to value your home, get a real estate appraiser, who will look at recent market sales in your area, and adjust for nuances of your property. To value your loan guarantee, you would need to figure out what it is actually worth to the business, which may be difficult.
In a perfect world, you would be able to ask the bank to tell you the interest rate you would have to pay, if the loan was not guaranteed. This would show you the value you are providing to the business by guaranteeing it. ie: if the interest would be $100k a year unguaranteed, but is only $40k a year guaranteed, you are saving the business $60k a year. If the loan is to last 5 years, that's a total of $300k. Of course, it is likely the bank simply won't offer you an unguaranteed loan at all. This makes the value quite difficult to determine, and highlights the underlying transaction you are considering:
You are taking on personal risk of loan default, to profit the business. If you truly can't find an equitable way to value the guarantee, consider whether you understand the true risk of what you are doing.
If you are able to determine an appropriate value for the loan, consider whether increasing your equity is fair compensation. There are other methods of compensation available, such as having the company pay you directly, or decrease the amount of capital you need to invest for this new set of equity.
In the end, what is fair is what the other shareholders agree to. If you go to the shareholders with anything less than professional 3rd party advice (and stackexchange does not count as professional), then they may be wary of accepting your 'fee', no matter how reasonable.