My main fluxing (sawdust) is done in the dutch oven smelting pot. Resulting in pretty clean ingots. I will flux again with pea sized balls of bullet lube at the start of the casting (ladle) session, only. I use two tablespoons..........one standard and the other with slots cut into it. I do it the way Rick describes...............bring alloy up from bottom and pour it though the bullet lube/sawdust. Starting off with the standard spoon and finishing with the slotted one.
When casting, sprues are dropped back into the pot, soon as they are cut. They don't have time to oxidize. I run my pot temperature a little higher to compensate for the slightly cooler sprues. Rare occurrence to actually have to empty the pot to clean/scrape.
At the end on my session, I empty the pot to roughly the one inch level. Mainly, because I don't trust them to work the next time I use them. Especially, the Chinese made RCBS Easy Melt or the Lyman Mag 25 furnace. I had the Lyman PID fail during a casting session. Turned out to be a blown mini fuse inside the pot housing.
I regularly, return clean rejects (bullets) back to the pot. No need to float a piece of metal..........that would interfere with the ladle. Reclaimed bullets are remelted during smelting.