I am interested in induction brazing small parts (OD less than .25 ")

...and I was wondering how the coil design, ( ID of coil, number of turns) affect the heated zone and what is the best way to control the heated zone. Is there a rule of thumb to get the best consistency out of the heated joint?

Proper coil design operating at the correct frequency should easily control the heat to your small part brazing application. It is recommended that you seek application consulting advice to set the process up correctly.