Calculated properties

Currently, I don't see any possibilities of having a number field performing simple calculations based on other number fields of the same object. For example, if I have a "Receipt" object, it would be great to have a number field that would calculate the tax automatically (ex: "Tax" number property = "Total Cost" number property X "Tax Rate" number property - this is a simple example to illustrate the request).

Similarly, it would be great to have calculated fields that span all objects. For example, the sum of all "Tax" number property of all "Receipts" objects.