What is the legal test for damages for breach of a contractual guarantee?

And further at pp. 348-49: "It follows from the legal nature of the obligation of the guarantor to which a contract of guarantee gives rise that it is not an obligation himself to pay a sum of money to the creditor, but an obligation to see to it that another person, the debtor, does something; and that the creditor's remedy for the guarantor's failure to perform it lies in damages for breach of contract only. That this was so, even where the debtor's own obligation that was the subject of the guarantee was to pay a sum of money, is clear from the fact that formerly the form of action against the guarantor which was available to the creditor was in special assumpsit and not in indebitatus assumpsit: Mines v. Sculthorpe (1809), 2 Camp. 215. "The legal consequence of this is that whenever the debtor has failed voluntarily to perform an obligation which is the subject of the guarantee the creditor can recover from the guarantor as damages for breach of his contract of guarantee whatever sum the creditor could have recovered from the debtor himself as a consequence of that failure. The debtor's liability to the creditor is also the measure of the guarantor's. "Whether any particular contractual promise is to be classified as a guarantee so as to attract all or any of the legal consequences to which I have referred depends upon the words in which the parties have expressed the promise. Even the use of the word 'guarantee' is not in itself conclusive. It is often used loosely in commercial dealings to mean an ordinary warranty. It is sometimes used to mis-describe what is in law a contract of indemnity and not of guarantee. Where the contractual promise can be correctly classified as a guarantee it is open to the parties expressly to exclude or vary any of their mutual rights or obligations which would otherwise result from its being classifiable as a guarantee. Every case must depend upon the true construction of the actual words in which the promise is expressed. "In the instant appeal, however, the actual words used simple, unambiguous, and contain no qualification except to impose a limit upon the guarantor's maximum liability under the guarantee. "The particular obligation of the debtor of which performance was guaranteed was to satisfy an existing debt to the creditor by instalments to be paid in the future. It arose from one of a number of interrelated mutual promises made by the debtor to the creditor and by the creditor to the debtor all of which were contained in a single contract; but the guarantor did not guarantee performance by the debtor of any obligations arising from any of his other promises. As between the debtor and the creditor all their obligations to one another to which their mutual promises gave rise, including the particular obligation of which performance was guaranteed by the guarantor, possessed the characteristics which the law ascribes to obligations whose common source is the same contract."