If starting your flush in dry soil you need to soak it first and let it sit for a while to let all the salts dissolve so they can be flushed out easier. Overwatering becomes a problem when old water remains in the rootball that is devoid of oxygen creating stagnant conditions that promote root rot. Fresh water used for flushing has lots of oxygen in it so really you can flush if it's wet and it won't really make a difference.
I try to avoid flushing ever as it's really a last resort when you can't fix what's wrong any other way or have overfed your plants and want to stop the burn. Usually just a couple liters of extra water is enough to flush the excess nutes out and stop the leaves burning. A few of my current plants started burning but using just RO water for two waterings was enough to stop it.