Why would you have to flush the Sledgehammer out? It's just yucca extract. It's saponin extracted from yucca. You can buy a less watered down version of the same stuff for 1/4 of the price. It might not have a cool name like Sledgehammer but it's the same stuff. It's just a wetting agent. There is nothing special about what's in Sledgehammer.
And it doesn't matter if it's a photoperiod plant or an autoflower plant. There is no need to be flushing unless you seriously overfeed due to inaccurate measuring.