In a slightly more complex situation, I decided to use AJAX to make sure the calculation is only done in one place, i.e. on the server. Coding the same calculation twice (on the server in C#, on the browser in JavaScript) is code duplication and might negatively affect maintainance; making use of a property of the calulation like you proposed seems even more dangerous because once someone decides that from now on, the formula has to be
B = A * 1.05 + C
your code will create subtle errors and probably cause a lot of headaches.
