They key to flushing is to have the plants on the right balance of nutes in the first place. When you remove or reduce the feed it should be visible. You want the plant to use up the excess stores of nutes. Too many people focus only on the medium, but it's not the coco you'll be smoking so I honestly can't get my head round that bit. knowing when your buds are fully set and formed and being patient enough to let that happen plays a big part in knowing when to flush as it does knowing when to harvest.
I allow 30-40% runoff every feeding in a top feed drain to waist system and flush by dropping the ppm's to like 300 for the last week.
Stick up their ass and ass mouth, wow there is some sheep thinking haters here. I don't flush and I will Pepsi challenge mine against anyone on this board. A proper grow and good cure is where your flavor and smoothness comes from. Starving your plants for two week in their peak grow time is not a good idea IMO. I have 10 yrs experience and I have done numerous side by sides, so I'm not just speaking out of my ass mouth either
A proper cure won't get rid of the excess elements in the weed though, so reducing the amount of residual phosphorus in the product before it is chopped is a must in my opinion. That doesn't necessarily mean staving the plants for 2 weeks either. A proper flush is just allowing your plant to use up its excess.