Hi Rolf,
Your oven is the way to go. I would ignore the lead bath or oil method. However, I find 750o F works best for most springs. I have now made a lot of springs and re-hardened and tempered others. Using my current regime, I've not broken a single spring and made good springs. Some time ago I experimented with mainsprings made from 1075 steel and tempered at about 700o. A couple broke but most were fine. I now prefer the higher temperature as it seems to be almost fool proof even with cast springs.
dave