What is the test for subrogation in insurance law?

The extent to which this right of subrogation will be taken is set forth at page 389 where Brett J.J. in Castellain v. Preston (1883) L.R. XI Q.B.D. 388, states: "In order to apply the doctrine of subrogation, it seems to me that the full and absolute meaning of the word must be used, that is to say, the insurer must be placed in the position of the assured. Now it seems to me that in order to carry out the fundamental rule of insurance law, this doctrine of subrogation must be carried to the extent which I am now about to endeavour to express, namely, that as between the underwriter and the assured the underwriter is entitled to the advantage of every right of the assured, whether such right consists in contract, fulfilled or unfulfilled, or in remedy for tort capable of being insisted on or already insisted on, or in any other right, whether by way of condition or otherwise, legal or equitable, which can be, or has been exercised or has accrued, and whether such right could or could not be enforced by the insurer in the name of the assured by the exercise or acquiring of which right or condition the loss against which the assured is insured, can be, or has been diminished. That seems to me to put this doctrine of subrogation in the largest possible form, and if in that form, large as it is, it is short of fulfilling that which is the fundamental condition, I must have omitted to state something which ought to have been stated. But it will be observed that I use the words 'of every right of the assured'. I think that the rule does require that limit."