How to determine reliability of a change in an arbitary metric

I have been using a/b testing in order to optimise changes I make to my websites. Previously, I have set a single conversion goal and have ended up with a conversion rate for each group. I then used R (with the power.prop.test function) to calculate the power and significance of the change in conversion rate to decide when it is safe to stop the test and be confident that the values have meaning.

I am now able to get information on a wide range of metrics about the site for each group. However I am not sure how to do the same test for an arbitrary metric - e.g., revenue. Could someone point me in the right direction? I am happy to use R so just pointing me at the right set of functions would be great, but I do like to try and understand what I am calculating so any explanations or links to useful site would be even better.

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