It is not really possible to accurately determine the CPU temperature with the variables identified. The problem is the interface between the CPU and the waterblock. That is a large variable, and if you are going through the effort to do this, and not just read the on-core thermal diode, you are going to need to accurately account for it. The effect of the thermal interface is determined by a number of factors you did not specify, like mounting force, and TIM thickness (which would be a function of mounting force). It would get very complicated, and therefore expensive, to get a more accurate reading than the built in thermal diode (it probably isn't even possible).