I believe that was the purpose of the previously mentioned lead bath, evenness and consistency of tempering heat. Another thing that can come up with tempering is to hold the temp for some extended period of time can be a good idea.
The kiln may be a pretty good option, if you can monitor and control the temperature. If the kiln has uneven interior temps, which is likely, then placing the spring in simply a piece of pipe in the center can do the job of evening out the temp that treats the part.
Chances are if the spring breaks, at least part of it was too hard. It may matter if the piece got to the fullest hardness it could, to be able to temper it back consistently. Charts are easy to find for figuring out temps. Being able to judge colors can narrow temps down, but it may not be the easiest thing to figure out without a pretty good bit of practice.
