The ideal temp will be the one that is just enough to get the job done! Lead (pure) melts at about 330C. Make absolutely certain that the mould (or mold if you are American
) is dry as a bone and not of a shape that will trap pockets of air - I've made one or two
mistakes like this. If possible, heat the mould before pouring.
The lead temperature will vary from job to job. It will need to be hot enough to pour and to not solidify when it meets the cooler mould, so it really depends on the exact process you intend to follow, and how big and hot the mould is before pouring!
Play safe - wear eye protection!
Doug