You will never get an exact correct prediction....you have to understand that there are too many individual variables as well as long history for that tort that will dictate size/temperament and a host of other pieces that make up a tort....you can not enter this information into some computer based program....
Signs of a healthy tort are clear clean eyes, plump juicy looking skin, a solid/heavy feeling tort when picked up, healthy appetite and attitude....so many other factors that are not of size base...