If you glue it in loose (with a little slack, I mean), you can tighten up with a match, cigarette, or incense stick...
Just glue into place, leaving a little slack, like I said... Then move the heat source closer and closer to it, and when it all lines up, the sprue will suddenly go taut...
As for timing, I just hold it with one hand over a candle, then when it curls up or droops under it's own weight, it's ready to pull....