Professional alloy people say not over 10% lead shot to keep a low toxity level. Also, it is wise to keep the alloy temperature low as practicle for good casting, to minimize vaporization of the alloy components.
I make it a point to never breath fumes off the lead pot, no matter the alloy, so would use however much I felt like. I advise all casters to do their casting with a good consistent breeze on their back so all fumes are carried away. In my mold shop, where I keep the pot going all day every day, on high temp for testing molds as I cut them. The high temp causes quite a bit of vaporizaton, so I've built a sheet metal shroud which covers back, and two sides of the pot, with just enough room to access it easily. The Vent pipe is 6 inch through the roof, with a booster fan which I use sometimes, and always when I flux. I have a cone over the pot which the vent pipe draws from.
